first, go to mutation mode in the main menu. the default mutation setting is called "last man on earth", this is a mutation where only you and special infected exist. however, you can only play this mutation on campaigns. there may be other mutations similar to this.

there are multiple mods that allow you to play a game without bots. you can find any of these mods by searching "no bots" in the workshop.

here are the best mods for this which i will link right here:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2710762847&searchtext=no+bots

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2814992394&searchtext=no+bots