Just for the record, the primary poling engine alone can poll as many elements as an additional polling engine ie: 12.000 elements. Therefore, if you're only going to be using 8 of the 9 polling engines then you'll end up with a polling capacity of 9,600 elements total, not 120,000. You will only be able to reach 120,000 or more if you
1. Have Additional Polling Engines (more than 8, or 9 if you use the Primary Polling Engine to poll as well)
2. Tweak your polling intervals to allow polling of more than 9,600 elements
Remember that the Primary Polling Engine a.k.a PPE a.k.a main orion server can not poll more than 12,000 elements with default poll settings.
There are ways of quantifying how many active polling engines you'll need to do the job but in general, and like with everything else IT related, prepare for more and build to scale before you begin.