We have a dynamic site & I'm planing for URL rewrite
But first of all I need to know how many pages in the site
Can someone direct me to any tool or website that can count pages?

Mucho gracias!

You may be able to do this with this free online tool called Statcounter. Just google it

StatCounter currently offer the following stats: Configurable Summary Stats, Magnify User, Drill Down, Popular Pages, Entry Pages, Exit Pages, Came From, Keyword Analysis, Recent Keyword Activity, Search Engine Wars, Visitor Paths, Visit Length, Returning Visits, Recent Pageload Activity, Recent Visitor Activity, Country/State/City Stats, Recent Visitor Google Map, ISP Stats, Browser Stats, O.S. Stats, Resolution Stats, JavaScript Stats, Email Reports, Multiple Site Management, Public Stats.

I tried several websites but everytime I have different results.
Is it because our web pages are generated dynamically?

Well if you do not believe in softwares/ other services then i have got an idea, do it manually because in this way, there's a zero chance of error in counting. Ok, i was kidding don't take it serious, I dont know then why are you having this problem.