Off the top of my head - when you create a site you're automatically added as a member of the site. If you're using an admin user to create all those sites, you're admin user will end up being a member of every site, which could cause your admin user to become really slow. You may want to check and make sure this is not happening for your site.
To answer your question, there's no limit. Also, for such a large deployment, you may want to seriously consider
Liferay EE so that you can take advantage of the performance improvements available in the EE version.
