Best practices, relative numbers of masters, slaves, caching only servers?

[Joe Bloggs]

Can anyone here offer comments/suggestuion inre what's considered
Best practice,,  inre relative numbers of masters, slaves, 
caching-only servers?     

We have three sites across the country, and a small number 
of systems (30-40).    the DNS is for intenal use only. 

but we could possibly have extended  outages on the site to site
links, and/or reasons to take systems offline for long periods. 

Would one (or 2) masters at each site (the rest slaves) work best?

any  disavantage/advantage to making all systems which 
are not the masters into slaves?   

or conversely, apart from the masters, perhaps configure 
just a few slaves at each site, with the rest, caching-only.  

or, fi we're talking small numbers of systems (30-40) 
does it matter much?