multi wordpress sites with same domain

need to setup couple of wordpress sites with the same domain, the folder structure look like this
folder/
folder/base
folder/site1
folder/site2
...

the mapping btw. sites and folder
example.com/site1 => folder/site1
example.com/site2 => folder/site2
...
example.com => folder/base (ie. any other request will go to this site)

each of them will be separate wordpress site, have difficulty to route traffic to different site, need help on this. Do not want to use wordpress multisite setup as I'll have to move some site to separate server when it grow big