It doesn't matter how many heroes you have collected you will still only be able to choose four of them for your team. To begin with the heroes you need to consider are those that are SR ranked regardless of their type (the order is B, A, R, S, and SR which is the higest ranked). You can get SR rank heroes by creating them via Hero Fusion, from Diamond Chest, Summon Shop, and Guild Point Shop (SR Ticket). When you start accumulating more heroes you can then start to focus upon their type. The following is a list of the five types of characters in the game:
These heroes are support units that defend other party members by using their skills.
These heroes can heal other party members or restore their health.
These heroes are ranged units and damage dealers.
These heroes are attackers that focus on damage dealing, usually through melee combat.
These heroes are damage dealers, usually from afar and through the use of magic.
The most important thing to remember when putting together your team is to make sure you balance it by adding one tank character (for defense), one healer (to heal party members) and two more (rangers or assault). You can always find out what a hero's role, power, and skill is simply by tapping it.