The Booty Royale Omnibus series has 2 Tankobon in each Omnibus.
They’re numbered:
1-2 (in core this is currently #1)
3-4 (in core this is currently #2)
5-6 (in core this is correctly #'d as 5-6, however, there is also an erroneous entry/picture for #3 that is a picture of the solo japanese tankobon volume #5, NOT the omnibus)
7-8 (in core this is currently #4)
9-10 (in core this is currently #5)
11-12 (pic missing, here is one:
#6 in core is an erroneous entry, it has a picture of the solo japanese tankobon volume 11
13-14 (in core this is currently #7)
Recommendation: Since you have a #5-6, you can put a dual #, so I’d suggest numbering all these with the proper dual numbering to remove the confusion of calling the Omni with vols 3-4 as #2 (for instance).
Hopefully not too confusing… looking at the pictures should make this kind of clearer.