Try
https://aternos.org/en/. It's free, and it can have unlimited slots. It also supports plugins. Though, I was playing with three friends and going through a Jungle and it took a bit for chunks to load, it wasn't my game either because it happened to everyone. Be wary of slow chunk loading. There is also
https://serverminer.com/ It is paid, you can buy the stone package for $7.58 per month. The stone package supports 15 players, all mods, and 1536MB of server RAM. Hope this helps.
