I think there are 4 assassins.
First one activated in the first day, and 3 afterwards.
The reason i'm saying that first one did not get a contract next day is "Once activated they will periodically be offered contracts."
We can't possibly know that period is 1 day, in my opinion it's at least 2 days, or maybe 3. We may come to a conclusion tomorrow.
It's random. All you do know for certain is that there are
at least 3
activated assassins in play currently.
It's hard to know if the one from the first day is the one that killed froodz or not, though they did both use guns while the other two used poison and likely some sort of sharp cutty-thing.
At this point you don't really know if each assassin has a unique weapon type (e.g. gun, poison, melee weapons) or even more specific (handgun, sniper rifle, cyanide, surgical precision).
You'll just have to wait and see how subsequent contracts are carried out.
It's also worth mentioning that you have no way of knowing if the assassins activated currently are even all of the assassins on the board as there's no telling how many are inactive.
Here's an interesting thought that comes across my mind... If the paladin/nun, the assassins, and the vigilante are all dead and then the mad scientist revives the shapeshifter should that count as a shapeshifter victory seeing as to how the victory conditions are met?