Limit on number of users in postgresql?

[Mark Walker <furface@xxxxxxxxxxxx>]

I want to make a client application that logs onto a postgresql server 
with an individual postgresql user role for each user.  This is as 
opposed to having one user role for the entire app, storing that 
password on the local client, and using the client app to do 
authentication against a a table in the connected database. 

I'm wondering if there's a manageable limit on the number of users in 
postgresql.  I could have conceivably thousands of different users for 
any particular app and multiple apps in the same server.  I would 
distinguish between different users for different apps by something like 
a user encoded name myapp_myname where to the individual app that would 
be user myname.

This seems like a persistent problem for a lot of database apps.  Is 
there any simple way of dealing with this problem?  A lot of apps store 
root passwords to the database locally, which is very insecure, 
especially on desktop applications.  It seems like a big headache to 
declare users globally to the server instead of at the database level.  
Am I doing more work than I need to do?

Thanks.