The 'upload rate' is to cap how much bandwidth used for uploading. If you leave it at 0, it will upload at maximum capability (based on bandwidth and if you have any other applications also using u/l capacity). I generally set it a 50 so that I can continue to run the FTP server at the same time.
The 'max uploads' refers to the maximum number of uploads you wish to allow simultaneously. Personally, with the bandwidth I have, I find that one at a time is best. I think that this must be a feature more attuned to a multi-user environment.
And from a link at Etree:
"The main advantage of being able to cap the upload rate is that people with capped DSL or cable connections can set their limit just below their actual limit, therefore avoiding saturating the upstream completely, which could make it impossible to surf the net or download at decent speeds. Remember that setting your upload cap too low will lower your download performance"