is it better to have equally high CP pokemon or 1-2 super high pokemon?

Depends how you play the game really. For some raids, it might be useful to power up one or two (or three or four) to the point where you hit the relevant Pokemon's final breakpoints, but if you're just attacking and defending gyms there's no real need and a bunch of L30 would work fine.

These days I'd say two max is the better pick.

For a team of six you can always find something else. For example for Alakazam you can always use 2 max ttar and substitute mewtwo, scizor, gengar etc instead of using a team of 6 L30s

I max one of each if IVs are good and several of the same species if they have good utility and resources are available to me. For example, one of each legendary because I barely make enough catch candies to take one to level 30+ after each wave (although, this time I might be able to max 2 rayquaza without using rare candies), but I have only maxed one Lapras, one Ttar, one Scizor, etc. I have 3 maxed Dragonite just because they are good all around and several Ttar and Rhydon above lvl 30, evolved "as is" from weather boosted catches. It's not a matter of preference to me but rather a matter of what I have available at the time.

2 maxed for sure.

Max out more pokemon.
All these people who are powering "a few of each to level 30" in order to have a few areas vaguely covered are wasting stardust. When the time finally comes to battle player vs player, are you going to send six level-30/35 pokemon at my six level 40 pokemon? GL.