The maximum amount of business processes that can be run simultaneously in the background is 5 for each application (for example, if you have two nodes with two apps on them but one database - the number of business processes will be already 10).
Alternatively, you can run business processes not only in the background but also "live" (which means you will see the uploading mask in front of you while business processes are running). This way, you can run plenty business processes at once but please take into consideration the memory capacity of the machine