In this case, you still need to decide how many users you want to simulate. Then, you can set the pace at which the transactions are executed by using the pacing function.
For example, you may want to generate 3000 Purchase transactions per hour using 1000 simultaneous Virtual Users. This means that each Virtual User must carry out 3 Purchase every hour. That is to say, each Virtual User must execute a Purchase transaction every 20 minutes. To do this, just set a pacing of 20 minutes for the Purchase Container.
For more information about pacing, see Container<in_pdf_design>.