Summoners are a big part of the strategy in Steem Monsters, and all splinters have at least two different summoners at this point. Some people don't really get the benefit to leveling them up (by combining multiple of the same card) since they don't get any stat increase, but it's actually very important to level them up.
Summoners can all be leveled up to level 6 or 8 (for Epic and Rare summoners), and by leveling them up you get to play more powerful cards along with the Summoner. At Summoner level 1, you can only summon level 1 versions of all the other cards, except for the Epic summoners that can summon level 2 Common cards.
Anyway, please see the tables below for an Epic and a Rare summoner to see which level cards they can summon at what levels. You pretty much want all Summoners to be at least level 3 or 4 if you want to play cards at a decent level, which would allow you to play level 3-5 minions.