Sky,
each page should target a particular group. Your meta tags let the engine know what's on the page. There really is no max. just make sure that what you have in the metas is really on the page / what the page is about.
An example is this lady's site. She wasn't listed in the engines at all, I told her to make html pages that linked into her cart and list all the products she sells there
For example - hippie soap
http://www.google.com/search?hl=en&q=hippie+soap (she only has number 4 now in google - but as she adds products it will go up higher - isellsoap.com).
But individual items are listed example - peace sign Soap:
http://www.google.com/search?hl=en&q...p+&btnG=Search
not bad number 2.
"60's 70's soap" not bad #1 , "soap peace sign" or "peace sign soap " "tie dye peace soap"
each product should be listed in the metas (so someone looking for it can find it), along with the category. then in the cart each should have their own metas....and on and on