Remote agents and local agents
Other than other Atlassian licenses, Bamboo is not geared towards physical users, but towards development capacities within the system, so called remote user. The more agents you have, the easier you can divide long test scenarios into parallel running batches and simultaneously run multiple builds. The more builds you use simultaneously, the quicker your team will receive feedback to the changes made (and the quicker you have the chance to resolve possible problems).
To find the optimum number of remote users, you should inquire the frequency of committed changes, the amount of build plans and the duration of the plan.