Your Marantz pre/pro is a 13.2-channel surround sound processor. ...With the additional option to add two more speakers to make it 15.2 (but only a maximum of 13.2 channels can operate all together @ the same time).
The two extra channel speakers are simply an expanded option in positioning, giving you more versatility, flexibility in experimenting, etc.
Your pre/pro is quite sophisticated and with many desirable features for a full fledged high caliber home theater surround sound system...Audyssey MultEQ XT32 (Platinum), Dolby Atmos, DTS:X and Auro-3D. ...Plus so much more that it would take a book or two.
If your goal is say to have a 7.2.4-channel system (11 speakers plus two subs - 7 main speakers, 2 subwoofers and 4 height ceiling speakers), an amp like this (5 channels) would do:
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https://emotiva.com/collections/amps/products/a-500
With your 7-channel Marantz amp you'd have 12 power amp's total, one extra.
If you want to go the full capability of your Marantz SSP (pre/pro), you would need another mono amp, or don't get that 5-channel amp above and look for a 6-channel power amp instead (more rare), like this one for example:
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https://www.amazon.com/6150-Home-Theater-Reference-Amplifier/dp/B001GL0K7K/ref=sr_1_17?s=tv&ie=UTF8&qid=1550950189&sr=1-17&refinements=p_n_feature_keywords_two_browse-bin:7306173011
That would allow you to go full steam for a 7.2.6 channel setup or a 9.2.4 setup.
The first number (7 or 9) is the number of speakers @ ear's level (or floor speakers, with the surrounds not more than say a foot above your ears (tweeter), the second number in the middle is the number of subs (2), and the third and last number (6 or 4) is the number of height (in-ceiling or on-ceiling) speakers.
I don't know if you knew that or not, but I mention it anyway.
And in the Marantz manual (online) you have the various configurations with graphs.
I recommend that you take your time and read it all.
* In general people purchasing products of that caliber have some basic knowledge on setting them up. But I also know people who buy class audio/video products without much experience. They want the best they can afford and usually pay an expert to set it all up, like an audio/video installer.
We're here to help you, for free. We give you the instructions that you need to set up your home theater in simplicity to your taste.