When I play, I play with two mates. However, sometimes some 'lagg' occures when rendering new terrain/more zombies. I am running MP on my own PC (i7 4790K stock, 16GB DDR3) and increasing will make this worse for me I think.
I did some research and hosting a server at a proper hostingcompany, isn't that expensive. It seems normal for me that those servers won't lagg as much as when I host it myself, so when we start hosting, it would be awesome.
However, when wandering alone and you want to loot something, but a pack of zombies are following you, you wouldn't want even more. That's in our case though, since we yet at day 13. With proper weapons, more zombies would be the same challenge as the current amount with bows and melee weapons imo