Re: how many application server licenses is good to get for a 25 userdb?

A few things to consider:

• The App Server can (theoretically) be used for non-database work. Therefore, the App Server licensing does not need to match the database.
• By Oracle licensing rules that are clearly [sic] stated on their web site, DB user licenses are 'Named Users'. Oracle does not use Concurrent users, and therefore any App Server based multiplexing or N-1 concentration could violate the license rules.
• You might want to discuss licensing alternatives within your org's accounting policies. I ran across one org. that considered leveraging a Named/Term followed by CPU/Perpetual once they were comfortable & properly staffed.
• Last time I checked, no Oracle product enforced an upper limit licensing count within the technology. Most major s/w vendors reserve the right to work with the customer to periodically re-evaluate the proper license count. A good set of license management policies and associated methods of managing license metrics is highly recommended.
• I strongly encourage you to look at the other things that the App Server includes. For example, App Server includes access to Workflow, Queueing, LDAP, a variety of integration connectors, and so on. Right for your organization? Only your org can decide, but only if you know it's there and included in the package.