The standard limits are:
- 10,000 keywords per ad group
- 100,000 keywords per account
- 20 ads per ad group
- 10,000 ad groups per campaign
- 10,000 campaigns per account
- 3 accounts per credit card
The difference match types for a keyword (exact, phrase, broad, and content) aren't counted as a separate keywords. In other words, one keyword with multiple match types counts as a single keyword against your account limits.
As for the number of accounts necessary to accomodate 500,000 keywords, 500,000/100,000=5. There are options, however. If you're fielding that many keywords, you may qualify to extend your account limits. Contact support for more information.
Hope this helps.
Charles
Micro Marketing