While I'm not obsessed with strict adherence to 5e D&D rules, the rules were very much made and balanced around the idea of having multiple summons (in fact, there are entire classes and subclasses who are based around the concept.) There are several iconic D&D spells that won't be implementable properly with this restriction - it's not a small, minor change, it outright guts a core mechanic and a major play style. Nor is it something that makes the slightest sense as a "tabletop to videogame" conversion - if anything, a videogame is more suitable to having multiple spells. The argument that it is easier to balance with only one summon is similarly nonsense (the original game already balanced having multiple summons; by restricting it to just one summon you are pointlessly creating more balance-busywork for yourself in order to make the game worse by removing things that were possible in tabletop.)
Beyond that, summoning multiple creatures is more interesting for players because it lets us try different combinations and interactions. It makes no sense to limit people to one summon at a time and then rebalance all the summons around this restriction that wasn't in the original mechanics, especially given that you're making the game less fun and spellcasting less interesting by going this route, while stripping entire subclasses of their main reason for existence.
In particular, restricting us to just one summon results in a brutally unfun interaction between familiars and unseen hand, since someone who uses a familiar cannot cast Unseen Hand at all without losing their familiar. At the very least, please make those not count against the summon limit - unseen hand is a limited utility spell, it's completely obnoxious and unintuitive to have your familiar vanish into the aether (wasting the spell slot used to summon it) simply because you cast unseen hand.
(Obviously spells like Find Familiar and Summon Companion the like should be limited to one of that type of summon. But you should be able to summon other stuff using other spells as well, otherwise players who use one long-term summon get locked into never using any other summoning spells ever again. How is that fun?)
Last edited by ZetaZeta; 30/12/20 06:57 PM.