A quick read of the document which is the original s106 agreement and the variation to it in 2012 shows that the limit is on ATMs and not pax numbers. The airport is limited to 53,300 ATMs annually and the forecasts by the airport's own consultants suggest that would equate to approximately 2m pax. The relevant paragraph is 3.41 on page 79 of the pdf document.
ATMs are defined as any fixed or rotary wing take off or landing. In effect an ATM need not have any terminal passengers (such as flying club activity). Someone with more time than me might want to model pax numbers based upon max atms in a year given the mixed nature of the aircraft fleet at SEN and taking into account average yields on those flights and discounting all non-pax flights from the calculation!