How much data can be stored in a ZIP file comment? I would like to use it to store information in XML.

They are limited to 32k. However, if you are going anywhere near that value, it would be better to add that information as a separate file instead.

What is the maximum number of files that can be stored in a ZIP file?

Same limit, 32k entries. The total length of the zip file is limited to 4 gigs. All these limits are related to the types of variables used in the zip file specification. Interestingly, there's a proposed spec for 64-bit extensions to the zip format, but that's not widely used yet, and there is a lot of debate still going on.