First, go to ruTorrent "Settings" (screenshot below).
Then, go to "Connection".
Over there, under "Other Limitations" you have the ability to choose the number of upload slots.
Please note: By default, these two boxes are set to 0 which is unlimited.
In addition, under "Downloads" section, you can control the number of upload slots in your ruTorrent.
RapidSeedbox preconfigured these settings to what we consider to be our best practices. However, you have the liberty to change them based on your personal needs.