New and clearly naive.
Number of punters at parties is greater than SPs.
It is bad form to hog one girl, never mind 3 or 4!
Occasionally parties don't attract many punters but like any business without enough paying customers this will not continue.
Girls do parties to try earn and earn more £/hour then normal.
OK there will be some "less attractive" SP who are there because they don't get so many bookings, but still means the same.
They can only do this if the ratio of men outnumbers the SP. If the numbers are not favourable for the SP then they will either cancel or reduce the number of SPs. Rarely towards the end of a party will you get 2 or 3 SPs willing to give you multiple action.