I think it's important to consider which team member really NEEDS which buff in order to be either protected enough or offensively powerful enough to contribute in the combat. I usually keep it to 1 or 2 plus irid resolve for xenos. I like knowing I can count on them firing in the first 1 or 2 turns.
Of course, if you have 3 on inits and one of those folks gets sniped to death before theirs fires, if the combat lasts 3+ rounds you'll wish you had 4 on init talents instead.
That is true when getting your soldiers sniped is part of your strategy. Otherwise you could use that talent slot fore something that you will actually use. E.G. to prevent your soldiers from getting sniped.
1 if your strategy relies on it. 2 or 3 if they are just nice to have.
Late game most combats dont last longer when 2 or 3 rounds anyway.
That depends on how your crew is set up. Combats vs. humans are over that quickly (or even in 1 round), but those are so easy your strategy shouldn't hinge on them. Personally, my combats vs. Jyeeta usually take longer than 2-3 rounds, but I have more of a generalist crew build (while still adequate to take out Jyeeta except on rare occasions), with a non-combat captain and officers in only 3 of the 4 combat slots.