We know about only a handful of Hands:
Mara Jade, later became a Jedi Master, married Luke & had  a son with him, Ben Skywalker
Shira Brie, later the Dark Lady of the Sith, she also secretly served as Vader's Shadow Hand & she eventualy became the Sith Master to Jacen Solo (Darth Caedus), killed by Luke Skywalker in a duel
Arden Lyn, a 25 000 year old Dark Jedi who awoke during the Empire Era, seduced by Palpatine to become his Hand by promising her he would resurrect her long dead lover Xendor (THE FIRST DARK JEDI in galactic history), 
Sarcev Quest, a failed Jedi Padawan, later the Emperor's Hand
Aralina Silk, she enjoyed killing both the Rebels & Imperials alike
Marek Stele, a pilot & Hand who left after the Emperor's death & later served as a pilot during the Yuuzhang Vong War
Cronal, aka Shadowspawn or Blackhole
Sa Cuis, used by Palpatine to test Vader by sending him to kill Vader. He died by Vader's blade
Jeng Droga, went insane after the death of Palpatine & later killed by Jedi Knight Kyle Kattarn 
Jallar Golin, he hated every living being, had a strong Force connection to the Emperor, was sent on missions to uncover the disturbances in the Force the Emperor sensed
Roganda Ismaren, also she was one of Palpatine's concubines, later killed by her own son
Vess Kogo, used only when brute force was neccessary

The Emperor always kept the number of his Hands to himself, not even disclosing it to Vader, Grand Moffs, Grand Admirals nor his sycophantic group of non Force-sensitive "advisers" like Yupe Tashu & who worshiped the Emperor as a near divine being. Only a handpicked group of individuals from the highest eschelons of Imperial Command structure knew about the existance & the authority of the Emperor's Hand. It was paramount that the Hands also never knew that there were others like them. Beliving (like Mara Jade) that she was the only Emperor's Hand. In truth it is never revealed in any book, comic or any other source material how many Hands operated under Palpatine. 
But we do know that there were over a dozen Emperor's Hands active at any given time. 
Except for Mara Jade, we only know the names a of few other Hands. (as writen above)