I have like 20-ish[0] people using my Migadu account and I've only hit the hard limit once, and it was because one of them was trying to do a mailing list....

Once I got them on a better path, I haven't had any issues. I don't remember what plan exactly, but I've been a customer for a long time, since they were founded pretty much.

0: I host a non-profit and we use my migadu account, since they are broke and I'm too lazy to count the actual number.