You should only put 1 keyword meta-tag on each page, with each keyword meta-tag containing no more than 12 (ish) keywords. It’s not a hard capped number, but remember that the more keywords you put in the tags, the more dilluted each keyword will become. Lastly, make sure that each page has a unique keyword meta-tag. This will avoid repetition and duplicity.

There are a few strategies for this:
1: Stemming
Use the main keyword for the page to generate secondary and tertiary keywords. Put the first 3 or so in the title tag, and put the 12(ish) keywords in your keyword tags.

2: Keywords + Branding
Use 2 keywords in the title tag and end the tag with "from your company". It’s not necessary to use your own brand in the keyword tag.

3: Shotgun method:
Use your main keywords to form the core keywords, then use action keywords like "buy x", or "x for sale", then use messaging keywords like "best x" or "x in bulk". This will cover most of the user search intentions.

4: Simple method
Out of 12 (ish) keywords use the top three searched keywords that are relevant (you can use the keyword popularity tool on Google or Yahoo). Then take the 3 keywords at 75% frequency, then 50% then 25%. This will cover your bases to capture the high value search terms as well as the smaller searched terms to drive traffic. It’ll take time to drive traffic from the high level keywords, but you should get fairly short term results from the lower level ones.

You got to have your main keyword.
And based on that main keyword, you can generate more keywords. Visit http://inventory.overture.com to produce more keywords.