You can summon total of 50 units in brave frontier but it's important to know that you will only able to summon 5 units every time and each one needs different type of items which you need to collect in order to do so. Therefore, instead focus on collecting different types of characters from 5 races including human, beastfolk, torimian, valkyrion and papulatus. They are said that each race have their own unique set of skills which you can check at their description when selecting what kind of unit do you want to buy. Furthermore, there are also units that can be obtained by visiting different worlds or by completing the daily/weekly bonus quests.