How to manage parallely process

Question

My situation is

I have to execute multiple  processes 

try to run ( 15 to 20 ) process parallely
and more than 20 requests starting making Queue for those request

As any request process gets completed so
take item in Queue to process in it and so on

In my mind i have 2 solution for that
1) wait call back 2) asyn call back

whats the difference between them

Should i prefer any of them or some thing else which is perfect  for me 

kindly suggest
thanks