there are a few mods that can do this.

ValheimPlus
OdinsQoL

both of these are comprehensive mods tho, with loads of options, increasing the server player count is just one of the options.

if you want a standalone mod that has its sole purpose of increasing the player count to servers, this one works. it only needs to be installed on the server, no connecting clients need to have it.

https://valheim.thunderstore.io/package/Azumatt/MaxPlayerCount/