When planning a GSM network there are many aspects that you must put into consideration like the frequency spectrun that is available to you which, so you can see whether you want to use frequency reuse and Frequency hopping, so if your freqeuncy is small there is a certain amount that you can not pass if you want to maintain quality of your network for every BTS e.g s222.
If I ignore the frequency issue, you need 50,000 X 0.02=1000 erlang. Every sector of a S444 can carry (as you mentioned)20 erlang x 3 sectors=60 erlang. Okay that means every BTS can carry 60 erlang. 1000/60=16.7 You need 16.7 S444 BTSs or 33.4 S222 BTSs.