There's no maximum number of pages. The maximum is based on file size. 

https://accountants.intuit.com/support/en-us/help-article/partnership/resolving-error-12031-e-filing... says:

The attachment size per PDF is limited to 1.5 MB, for a total limitation of 32 MB.

This means you can have as many 1.5 MB attachments as needed, but the combined size of the attachments can't exceed32 MB. 

While 32 MB is the largest combined size, you could experience issues e-filing a return at around 20 MB.