First off, mounting that many filesystems seems like a really bad idea - there must be a better solution to whatever problem you have?
Anyway, to answer your second question: an answer on a previous serverfault question seems to indicate that the limit is about 1 million mount points (2^20) per file system type for recent kernels, and you can mount at most 256 different types of filesystem.
For older kernels (pre 2.6) the limit is 256 mount points per filesystem.
Edit in response to the comments, I propose the following alternative solution:
Use XFS, which allows you to use project quota, a form of directory quota.
Create a separate XFS file system which will hold all webservice user data. Mount it at e.g.
/mnt/myWebService. Then, for each webservice user, create a project directory (eg
/mnt/myWebService/username1 etc) and set quota accordingly.
For instructions on how to set up project directories and quota, you can for example check out this blog entry or the RHEL XFS Quota Management page