The better question here might be "how do you purpose your web applications when you create them?" There really should be no hard limit as to how many web apps are created per farm. However I've seen many a person treat web applications as site collections and things get really out of hand when you do that...
My policies are this:
1) If a group of sites have an entirely different security policy (Forms Vs. Windows Authentication, Internal vs. External Access, etc.) you're talking a new Web Application
2) Entirely different branding is usually a reason for a new web application (to the point of where it's a whole new logo and webpage)
I usually try and discourage people from getting a new web application for URL vanity reasons or "but we want our-department.company.com instead of company.com/thisdepartment" but have been known to relent there. Some people have firmer governance in place that prohibits this sort of thing...