No one can afford, if one RAC setup for one application. All depends on how many databases you want.
In One RAC environment, you can create databases according to your lisence,
Each and every organization differs based on their requirement.
We have 1 RAC environment(3 nodes) and we have 5 applications altogether running in the same RAC
The RAC licensing will allow you to run as many individual databases as you wish, but that brings with it additional administrative overhead (multiple backups, patches, etc) at the expense of division of resources. If your applications can all peacefully coexist in a single database, and don't mind having the same maintenance schedule (if the single database needs to go down, all tenants are affected) then I suggest one "large" clustered database.