We support up to 10 attachments, as long as the total size of the message body text and all attachments is less than 5 MB.
Carrier also has limits. The average is 300kb-600kb and can go up to 3.5MB for tier 1 providers (AT&T/Verizon, etc.)
These image file types are supported:
Note: As of May 2021, there is an issue with Toll-Free MMS file size support; for these numbers, you must ensure that any attachments (including images) are 600kb or smaller.