Unfortunately there is no way to accurately calculate the number of people who are looking up your whois data. While some registrars might do this, there are databases that also record it.
If you want to find out who is snooping around, check out your weblogs and spend some time going over them, especially the referrers.
If you are concerned about privacy, you could always check with your registrar to see if they offer WhoisGuard (like namecheap) or Godaddy's Private Registration. It costs a bit extra, but its well worth it - cuts down on the spam too.