I just got Deus Ex: Human Revolution, and I'm trying to get a sense for how much flexibility I have in spending Praxis points. How many of these can I expect to accumulate in a playthrough of the game? Does this change for different difficulties or for different playstyles? For example, if I invest heavily in hacking, will I get enough of a return to make up some of the points spent?
I think I'm around half way through and I've found around 20.
A good rule of thumb is to always have a couple spare, so if you come across a five story drop that you have to survive or a level 5 security device or a wall you want to punch through, you can without having to look for another way or more praxis kits.
Edit: Because of the age of the game we can only estimate, but the general consensus is there are 22 to be found in the world and on a near perfect playthrough of the game 40 can be unlocked through experience. So 62. Just shy of the 68 needed to fill all slots.
My experience is also that you can fill everything short of 6 upgrades when the game ends.
Hacking generates a huge amount of exp as you play if you are willing to spend time hacking everything in sight. I ended the game with about 90 each of stop and nuke viruses, due to my hacking everything and not being willing to use them, in hindsight using nukes on any node with a level 4 or above rating should be fine. It also allows you to explore everything in the game more easily.
To help with the exp, also try to stick to pure nonlethal moves, use takedowns whenever possible and headshots with stunguns and tranqs when it is really neccesary. (Of course i did clear that huge crowd of guards coming in after me at of TY Medical with two gas grenades, but that was mostly disinterest in sneak-hunting the whole bunch.)
Augs you can safely skip are the two aim stabilizer augs, the cooldown timer aug, the land-quietly aug, the social enhancer aug, the enhanced radar aug, and finally all the hack analyze and hack fortify augs. That's a total of 12 non-useful praxis points you can save on, so on the bright side you can be assured of upgrading everything you want at a pretty early point in the game.