I don't have a specific number to point you to. However, I can point you to case studies. I pulled a list together and posted it on my
blog. You'll notice when flipping through these that the schools have taken various approaches to using Tablet PCs. Some schools have issued to teachers only, others use the older mobile cart style, while others are part of one-to-one computing initiatives. You'll also see that some school districts that have issued Tablet PCs to teachers and there are several hundred classes per school.
Hope this helps.
--Lora