You can start with 8 gb on one machine in different db/dir locations. Most providers allow the vps to upgrade to more ram or disc storage later.
So like -> [foo.domain.com] on nginx{} >localhost:5555 and [fee.domain.com] to strfy2 on [localhost:6666], a strfry router without own database on [localhost:7777] ...on the same dir location and/or another ...and so on and on... use nginx proxy/upstream/load balancing function..all as you like.