The way you limit how many files are transferred (uploaded or downloaded) at once is by setting
simult_transfers to something lower than the default of 10 in the
__main__ section of the
syncdaemon.conf configuration file, and restarting syncdaemon.
You can edit the system configuration file,
/etc/xdg/ubuntuone/syncdaemon.conf, or the user configuration file
~/.config/ubuntuone/syncdaemon.conf (or create a file there if one doesn't exist). I've found 3 is a good compromise value for when I'm on 3G; YMMV. If you don't have a user
syncdaemon.conf and don't want to edit the system one, you'd want to create a file that looked like
simult_transfers = 3
If the file already exists, you just add the
simult_transfers line without the section header.
Once that is done, you can restart syncdaemon by entering
u1sdtool -q; u1sdtool -c
in a terminal.