One of the problems with Genshin Impact is that, at higher levels, you need more and more experience items and ascension materials for characters, yet you only have a certain amount of resin to use for claiming these rewards each day. By the time you hit level 60, you’ll be grinding to get what you need to push your characters further. It’ll be even worse if you try and level every new character you get. This is why it’s generally better to focus on building your chosen team to a decent level, first.
Saying that, if you have no problem with a bit of grinding, you can level as many characters as you want. The only real barrier is resin, since you need it to claim mora and experience items from Ley Line Overflows, and to get character ascension materials from defeating bosses. But having two teams around is no bother at all, as you’ll actually need at least 8 decent characters if you want to do the later levels of the Spiral Abyss. Genshin is all about collecting new characters, after all, and if you actually want to use any of the ones you pull, you’re going to have to level them to match your world level anyway.