Abaqus speedup vs. CPU numbers

Hi

I am wondering how Abaqus speedup vs cpus numbers? Currently I only have 4 cpus. If I increases to 8,16 cpus, does it speedup 2 and 4 times faster? The tokens used for 4 ,8,16 cpus are 8, 12 and 16, respectively. I have one other application. The speed is almost the same after increasing cpus from 4 to 8. Thanks