Up to 10 sites per IP as long as they're in different niches/areas and aren't interconnected in any way. If you're building them as a network to pass back links to your sites, then as few as possible per IP (depends what you can afford).
I build networks of 250 sites and place them over 50 different C classes and at least 3-5 different A classes.
There are plenty of dedicated SEO hosts but most of them aren't great. They often over subscribe their shared SEO packages to cover their increased IP costs.
I use indianets and can recommend their 5 site per IP package.