How many exercises per workout?

Hello! your exercises per workout depend upon your workout program. Most of the bodybuilders follow 5-6 exercises per workout which target their all specific muscles. 
To make an effective workout program make sure that the exercises you are performing are effectively targeting your muscles or not.
For eg if you are training your chest muscles so you need a perfect workout program that hits all chest muscles (upper, lower, and middle pecs).
Try to add 5 to 6 exercises per workout to and make it an intense and effective workout. 
Hope this will help you.
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All the best and stay strong.
Cheers!