16 exchanges. Most products are multiply list meaning the trade ar more then one evxchange - not all,but most. So you broker can smart route to any venue even if there market is inferior to NBBO. You can't be filled at worse than NBBO with rare exception like flashing quotes. So the receive Mm can come to NBBO which is frequently the case. There can be NBBO ties. NBBO is the highest bid/biggest size - lowest offer/biggest size. So first recognize that NBBO can exist from multiple venues quote if the Mm don't want trade at those prices the order ships to NBBO and there is generally a added cost to ship. Lots of details on how NBBO works. Shipping can occur because of price and size. So might a broker ship to an inferior NBBO. Things like payment or your order being small. If your order is big your broker might to sweep multiple markets to fill. Another isue is hidden liquiditiy which would also acount to an order being sent to inferior market. You cannot fill ar worse than NBBO price, but not size
I believe the executing broker and or direct market participant must rely on the SIP to determine the NBBO as dictated by Rule 611 of Regulation NMS. Seems crazy to me since I have my doubts regarding the quality of the SIP feed vs proprietary feeds.