I myself like 10,000+ results for the term alone but lean towards available .coms more so. If its a single word Domain 5,000+ results. Results for Domain including the .whatever, 100+.
I try to stick to these rules but stray every now and then
Generally, I'd say 2,500+ for a .COM, and 2,500 - 5,000+ for any other extension. Also, I wouldn't go SOLELY by the OVT count when determining whether or not to register a domain name, IMHO. It is simply one other resource that can be utilized.
Hope that helps.