Source http://www.mysql.com :
Scalability and Limits
Handles large databases. We are using MySQL Server with some databases that contain 50,000,000 records and we know of users that uses MySQL Server with 60,000 tables and about 5,000,000,000 rows.
Up to 32 indexes per table are allowed. Each index may consist of 1 to 16 columns or parts of columns. The maximum index width is 500 bytes (this may be changed when compiling MySQL Server). An index may use a prefix of a CHAR or VARCHAR field.