shared album maximun people

You can only directly share with other iCloud users. If you are wanting to share with non-iCloud users, you have to create a public website for the album. Is that by any chance what is creating the limitation for you?

iCloud Photo Sharing - Apple Support

Get help using iCloud Photo Sharing and shared albums - Apple Support

Best,

GB

Actually, it says that the maximum is 100.

Maximum subscribers per shared album: 100 (the number of subscribers on each shared album)

This is from this support document. My Photo Stream and iCloud Photo Sharing limits - Apple Support

However, I've not been able to locate any troubleshooting for problems with that sharing.

Limits are not set by album, so not sure why you wouldn't be able to add another user who has an iCloud ID. So, not sure what the issue was. Has she tried re-inviting her son now that your daughter has been successfully added?

What, exactly, happened when the owner tried to add her originally? What prevented her from adding your daughter to the Shared album?

Best,

GB