Rather than asking how many Mandalorian warriors there are, I'm more interested to know, for a legendary warrior people with a history of standing toe-to-toe against Jedi in a war, just how much of the population is made up of actual warriors. Are we talking about an entire civilisation of warriors here?
Disney Canon – Just "Death Watch", a splinter faction
By the Clone Wars era, the Mandalorian culture as a whole is largely pacifist and has rejected their militant past. From The Clone Wars episode "The Mandalore Plot":
Prime Minister Almec: Mandalore's violent past is behind us. All of our warriors were exiled to our moon, Concordia. They died out years ago.
Obi-Wan Kenobi: Hmm, are you certain? I recently encountered a man who wore Mandalorian armor: Jango Fett.
Alemc: [angrily] Jango Fett was a common bounty hunter. How he acquired that armor is beyond me.
However, there is a splinter faction named "Death Watch", which idolizes these warrior ways and wishes to return to them.
Obi-Wan: Your peaceful ways have paid off. Mandalore is prosperous since the last time I was here.
Satine: Not everyone on Mandalore believes that our commitment to peace is a sign of progress. There is a group that calls itself "Death Watch". I imagine these are the renegades you're looking for. They idolize violence and warrior ways of the past. There are those among us, certain officials, who are working to root out these criminals.
We have no information about how much of a population the warriors of Mandalore made up before this exile, although certainly enough that the culture had a reputation as being warriors.
Legends – Pretty much all of them
Star Wars Insider 86 had an article entitled The Mandalorians: People and Culture, which describes them both historically and as a modern culture through the Clone Wars and Rebellion eras. I can't find a copy of the text, but here is a summary from Wookieepedia:
Males and females were on equal footing, although they often took different roles. Mandalorian males were all expected to be warriors, and were responsible for training their sons to be the same. Females were expected to have the same martial skills as males, and were responsible for the training of daughters. They were also expected to be able to cook, and to care for any young children and their home if the men were away. But if they had no children dependent on them, females would fight side-by-side with the men on the battlefield.
The article also explains that adoption was very common and genetic lineage fairly unimportant to the Mandalorians, so motherhood did not prevent a woman from serving as a warrior if another family wanted to adopt her children.
Note that The Clone Wars TV show is technically part of both Disney Canon and Legends. The fact that the Mandalorians were originally an entirely warrior race, then retconned to being an entirely peaceful one, was the main reason that Karen Traviss, author of this article and several books that heavily featured the Mandalorians, said she quit writing for Star Wars.
It depends on when and where specifically you're asking about, and if you include Legends soures.
In Canon materials we basically know nothing about Mandalorian culture before the Clone Wars. As of The Clone Wars the Mandalorians are pacifists under the leadership of Duchess Satine Kryze and her "New Mandalorian" movement. Because of this the number of warriors on Mandalore itself has been drastically (if not completely) reduced. Mandalore does have a police force, but we don't know what kind of training they have and if they are actually warriors.
When Mandalore became pacifists the warriors who didn't want to lay down their arms were exiled. We know at least one group (Death Watch) chose exile and went to Mandalore's moon Concordia.
In Rebels we haven't seen Mandalore itself but in newer episodes we have seen that Mandalorians have broken up into warrior clans and seem to be moving back to their warrior culture of old. Sabine's family, Clan Wren, have their stronghold on the planet Krownest.
In Legends materials the Mandalorians were pretty much all warriors. We see this in both the Knights of the Old Republic and the later New Jedi Order.
So, really we don't know what percentage of the Mandalorian population are warriors, bu we can assume it was higher in their past and at the time of Rebels, but much lower during the New Mandalorian movement that occurred before and during the Clone Wars.