25? That's the maximum allowed. So if you can fully use it then good if not, then still ok. What deck you wanna use, check your star count from the cards you are using.
The stars are usually found at the middle of the card, make a deck of cards with a combination of lit stars that amount to a sum of 25. cool bro ?
Honestly, that will depend on your deck and the cards you want to use. Normally it'll be a mix, 1 or 2 two/three stars and the rest four and five stars.
Well bro, in this game, u need defenders and attackers, a good deck would contain an equal amount of 4 attackers and 4 defenders. There ae cards that attack and defend very well... thats the reason they are cards that are expensive, got that? From there, its all up to you to strategic to play your own creation, deck building skills imo.
Of course there are players who go all the way out, aggressive without considering defending. So it depends on what you like, what's your style...