You should probably try to figure out your team's work capacity (i.e. how many tasks can each member do per working day) - you don't need to have a solid number and, per my experience you should probably level by the lower numbers. After getting to an average you can adapt it just remember that people must stick to one task at a time (moving unfinished tasks to "on hold" before they can take on another responsibility. If you're looking for a kanban tool to help manage your processes I could suggest the one from the company I work at, Pipefy.