The basic Help page says:
Each listing can have 5 variation details and up to 60 values for each of those details.
For example, if you're selling shoes, your 5 variation types could be color, size, width, material, and style. Each of those details can have up to 60 values: 60 different colors, 60 different sizes, and so forth.
You can have up to 250 variations in a single listing for no additional fee.
http://pages.ebay.com/help/sell/listing-variations.html