If I have to guess, I'd say about 10% of them are bots, but don't quote me on that number. This is because around every ten people I run into, one always exhibits many of the patterns and bahaviors that bots so often do.
If you're curious and want to see it in action, go to one of the popular leveling places (especially for lower-level players like Payon fields) and target the enemy that a player is about to attack; if that player immediately, like IMMEDIATELY, changes its target, it's probably a bot; and if you do it again to its second target and the player immediately changes enemies again, it's for sure a bot.