When configuring your load engines, Load Tester estimates how many are needed. The estimation algorithm tries to account for many variables in your test configuration, but tends to be rather pessimistic.
We recommend setting up one load engine and running your test with, for example, 100 users. The engines will analyze their own capacity and report that back to the test controller during the test, which you can see in the engines view. This will give you a baseline to determine how many engines you need. Note that the more users are running on an engine, the more accurate the capacity estimate will be. If you are running less than 10 users, the capacity estimate can be wildly pessimistic.