we do not have such guide because there are many factors to consider for this, not only the ones you mention
depending on the protocol you're going to use you will need a specific CUCM version, specially if you want MGCP or SCCP
if it's H.323 as long as the gateway has the H.323 capabilities you can use it
as for the number of users that is more related to the erlangs you need to determine what kind of router you need based on the PSTN trunks you will need to satisfy the users needs
you can search on cisco for ISR (integrated services routers) and you will get the series we have 2800, 3800, etc
each model supports N number of cards and the cards can support N number of trunks
if you have a CMM in a Cat 6500 you can add a 6 T1 module
you may also consider a universal gateway depending what you need and how much you want to spend
HTH
javalenc
if this helps, please rate
HTH
java
if this helps, please rate
