Are there immortals in both the X-Men and the Avengers? If so, how many per team?
While the Avengers have quite a list of immortal or near-immortal members (see list from "Is Thor the only Avengers that can't die?"), the X-men have had far fewer members of their group whose powers would include immortality as a by-product of their genetic mutations. However, many of the X-men possessed heightened regeneration abilities and correspondingly longer lifespans.
Juggernaut (Cain Marko) while he was imbued with the power from the magical Ruby of Cytorrak was, effectively immortal, since he had neither to eat, sleep or breath and was completely invulnerable to harm as long as he channeled that power. Now, the power of Cytorrak belongs to Colossus (Piotr Nikolaievitch Rasputin) it can be assumed that such immortality has moved to him as well.
When Cain Marko found the Gem of the mystical entity Cyttorak, he was empowered with magical energies and transformed into an immortal avatar for the entity in question. As the Juggernaut, Marko possesses superhuman strength, being capable of shattering mountains, lifting and using buildings as weapons, and extreme durability.
Jean Grey (or anyone acting as the empowered Phoenix) would have been immortal since she could completely reconstruct matter and energy in anything she wanted. As a result she could be effectively immortal. If and when the Phoenix force leaves Jean Grey, she would likely return to her human state, unless the Phoenix decided otherwise.
Immortal until Killed
Darwin (Armando Muñoz) recently returned to the X-men after the events of M-day whose power included constant reactive evolution, a power that allows him to evolve to counter any form of threat against him. While this does not guaranteed immortality, it certainly should mean a normal death should be outside of the realm of his lifespan but the upper limits of his ability are unconfirmed. Despite the movie, X-men, First Class abominable handling of the character, he has suffer near complete physical dissolution, been converted to energy and returned to human form.
While dealing with the shocking revelations of the truth about Krakoa and Vulcan, Beast found out that Darwin was still in fact alive. He was brought back to the Institute, where Beast determined Darwin was now a being of pure energy. While not conscious, he was shown to still have brain activity and was still alive, located at the X-Mansion. Once separated from Vulcan, Darwin's abilities caused him to develop a physical form once more.
Later (and at this point, his last evolution thus far)
During a battle with Hela, Darwin adapts to her touch by becoming a death god himself, Darwin has trouble coping with the after-effect of the god-like abilities that were granted to him. He decides to leave the team to adjust and find himself. Writer Peter David indicated on his blog that he would eventually return.
Beyond Human Longevity
Wolverine and Deadpool, both who possess superhuman and incredibly fast regenerative abilities that both allow them to recover from injuries that would kill normal humans, but also seem to slow their aging rate to a fraction of that of normal humans. So while they may not be truly immortal, they would certainly live lives much longer than normal humans (if their lives weren't as dangerous as they are, of course.)
Sabertooth (Victor Creed) possesses(ed) regenerative powers similar to Wolverine's. He also aged at a far reduced rate.
X-23 (Laura Kinney) a female clone of Wolverine who possesses a similar healing factor. She should also age at a reduced rate.
Longshot has an increased healing factor and can immediately recover from any injury instantly. As a genetically-engineered being from the Mojoverse, we have no idea how long he was designed to live.
Mystique (alias - Raven Darkhölme) has the power to alter her physical appearance and has an accelerated healing rate, though nowhere near that of Wolverine's. She does possess the reduced aging factor and ages slower than a normal human.
Fantomex also has an enhanced regenerative ability but it is not nearly as effective as Wolverine's so it is not known if it will included an increased lifespan.
Marrow, an X-man who used her bones as weapons and armor had an accelerated healing factor. She lost her powers during M-day.
Character noted to be de facto immortals.
- Wolverine* (Avengers, X-men)
- Deadpool* (X-men)
- Thor+ (Avengers)
*: Immortality via regeneration
+: immortality via other means
Character noted to immortals per se.
- Mr Immortal (Avengers)
- Firebird (Avengers)
- Hercules (Avengers)
- Wonderman (Avengers)
Wolverine isn't technically immortal as he just ages slower, so ipso facto he will one day die of old age. Juggernaut was immortal when he was imbued with the power of the crystal as he needed to neither eat, drink nor breathe. However the power of the crystal was transfered to Colossus so we can assume Juggernaut's immortality was also transferred.
All in all I think Wolverine is the best immortal because unlike the others he can't be killed by conventional means. The only way to stop Wolverine regenerating permanently is to burn him to ashes and scatter them to the wind... Wolverine rocks... so yeah:
Thor and whoever holds the power of the crystal is a true immortal.
Everyone else (Wolverine, Deadpool, Mystique, etc) just age at varying rates.
Cannonball (Sam Guthrie) may be an External, a rare subspecies of immortal mutants that also includes Apocalypse. Whether or not Cannonball is actually an External is not yet clear but it has, on at least one occasion, been shown that he apparently returned to life after dying.
Selene Gallio the first mutant and an eternal, older than apocalypse by several millenia. I believe she was something like 17,000 years old and all but indestructible. She had powers of telekinesis, telepathy, psycho kinesis, pyrokinesis, shadow manipulation, and the ability to drain the life-force among others. She awoke Magma's mutant powers and finally did in Kulan Gath long after Conan the barbarian and Red Sonja thought they did in ancient history. She watched the fall of Rome (and possibly had had in it). She was a master of both white magic (as used by Dr Strange) and black magic. Though she can most certainly be seen as evil she is undoubtedly the oldest and most powerful mutant to ever have been. She even at one time tried to sacrifice all of Rome so that the power she fed on from the thousands of lives could (and it seems would) have turned her in to a god. Although she was neither an X Man nor Avenger she was also an enemy of the X Force and ended up fighting together with some x men and even had her life spared from Rachel Summers when Wolverine interjected on her behalf.