The file limitation for each message is 20MB in total, e.g. if you upload 3 files with 7MB each in the same message, it will exceed the limit.
However, if the Pod has Expression Filtering version 3 (EFv3) enabled, the file limitation is 5MB in total for each message.
There is no limit to the number of attachments that are allowed in a chat room. If a user is unable to see some attachments that other users can, the chat room may not have New Members can see past content enabled.
To check if past content can be viewed by new members, navigate to Chat Details by clicking on the i in the upper-right corner of the chat and find Member Permission (Figure 1):
Figure 1 Member PermissionNote: If this option is not enabled, new members will not be able to view historic messages and attachments