ironically, or at least it seems so to me, Network Solutions seems to allow an unlimited amount of whois when other registrars won't. but that is personal experience, they do have a limit as I found this list on a website that seems to be helpful:
"Here are some examples of the limits set by different domain registrars. Please note that these are just a few examples. Almost all registrars have similar limits in place.
Tucows (whois.opensrs.net) rejects multiple simultaneous connections from the same IP. You will have to be satisfied with one connection at a time.
BulkRegister (whois.bulkregiter.net) will immediately block the IP temporarily if they notice a large number of connections being made. This is done by their software automatically.
Network Solutions (whois.networksolutions.com) will block the IP temporarily if you do more than 1000 lookups in a day.
Go Daddy (whois.godaddy.com) will simply start hiding all the relevant information from the returned whois text if your IP gets in their list.
Nominet (UK domains) has a new whois usage policy applicable from March 31 2003.
...most other registrars also have similar restrictions."
If you'd like to know where I found that just PM me.
Vivvy