You can use pre-1st turn stratagems as many times as you have command points to use. Anything that occurs during the game (post seize) you can use once per turn in general, some specifically are once per game.

How often can you use stratagems?

You can use different stratagems in the same phase, you can't use the same one more than once per phase. So if you reroll to hit during shooting you can reroll during charge, etc. you cannot use the reroll strat twice in the shooting phase for example. No it is only once before hand as well.

How many stratagems can you use on one unit?

Subject: Can you use more than 1 stratagem on a single unit in the same phase? you can use as many as you want. Only restrictions you have is you can't use the same stratagem more than once in each phase and how many command points you have.