You mean the number attachments or the total size? Sometimes I need 3 or 4 images to describe something, and then another for an actual disk file... maybe some source code. So I dunno, 6 items? Actually the number of attachments shouldn't be a big deal... why not an even 8 or 10?
As for total size... well would be handy to attach a .D81 but I guess that can be compressed... images are usually compressed already. Umm, 1.44 MB ? 2MB ? I don't think I'd try to upload anything bigger than that. Of course that's me... some people like to make docs in PDF format which can be a bit bloated. A windows utility like DirMaster might be a couple of MB. So maybe something more generous like 4 or 5MB ?