EXEDORE: Being able to sprint while cooking is a low level ability, and grenade shooting is a high level ability. As some have surmised, slots are not a concern with passive abilities that don't require d-pad activation.
H0RSE: So is there no limit to how many passive abilities you can bring into a match?
EXEDORE: It's only limited by your level, which dictates how many abilities you can buy overall.
taken from this thread - http://www.splashdamage.com/forums/showthread.php?p=243932&highlight=passive#post243932
Yes, you can get 20 abilities total, but you can only ever equip 3 active abilities at one time.
For skills that have upgrades (like turret 1, 2, etc.) you need to buy lower lvls before you can get the higher lvls, but the higher level skills replace the lower lvl ones, so you still aren't wasting skills/xp. However, the higher tirer skills are more powerful, so they cost more than 1 point to purchase.
http://www.splashdamage.com/forums/showpost.php?p=246478&postcount=6
http://www.splashdamage.com/forums/showpost.php?p=246512&postcount=12
Thanks for clearing that up.
This is where things get hazy, and I'm unsure.
If you only purchase "normal" abilities, that being, abilities that cost 1 point, then yes, you will get 20. But if you start to purchase upgrades (like turret 2 or 3,) abilites that cost more than 1 point to unlock, then your total might change. Like I said, it all depends on how the point system works.
Ex. say you get 1 point for every lvl - that gives you 20 points. If you are buying the more powerful, expensive skills, you are going to have less than 20 skills total.
Also it figures that most of the passive abilities will be general ones. So everyone can use them.