The train service requirement under the 2016 Franchise Agreement can be found in the zip file here
https://www.gov.uk/government/publications/east-anglia-2016-rail-franchise-agreement
TSR1 applies to May 19
TSR2 May 19 to May 20
TSR3 May 20 onwards
separately there is a separate STAR requirement effective Dec 18. That has already been delayed to May 19.
Table 21 of the TSRs shows 15 on weekdays direct Norwich to Stansted Airport services in TSR 2, increasing to 17 from TSR 3.
Like many things about the GA franchise we are waiting to see what will actually be delivered. As mentioned STAR is delayed, and personally I am very doubtful that para 105.2 will be complied with, although no doubt Network Rail mostly to blame.
105. Infrastructure upgrades
105.1 By no later than 1 May 2019, the Franchisee shall make all necessary changes to the infrastructure of the Stations (including relevant associated infrastructure surrounding such Stations) in order to accommodate the Passenger Services as set out in TSR2 and the Franchisee shall incur a minimum expenditure of [REDACTED120] in doing so.
105.2 Pursuant to paragraph 105.1 such changes to the infrastructure shall include such infrastructure works at each of Hertford East, Wickford, Manningtree, Kings Lynn, Elsenham, Ware, St Margarets and Enfield Lock Stations as are necessary to enable any Passenger Services comprised of 10 rolling stock vehicles to call at such Stations by no later than 1 September 2018.