What kind of stats are you looking for? Quest has some kind of SharePoint
auditing software. I'm not too familiar with it, I'm sure they've got some
information on their site. I use a free program called "Analog" to parse my IIS
logs. That gives me regular IIS usage  stats, like data in and out, sites hit,
refers, etc. We also use a free program called SPSiteManager to get a daily
report of how much data our SharePoint site  collections are taking, how many
site collections we have, stuff like that.

I will check it out. We are using some hardware for load balance...I
wonder if it can combine for multiple servers?

Can you post syntax and where we can download Analog?
I am interested on maximum # of concurrent users that visit our main
site - www.sample.com

Note that this site  are hosting multiple sites.
For example,

www.sample.com/sites/site1
www.sample.com/sites/site2
www.sample.com/sites/site3
etc.