Hi, frededrick mbogo!
1 TS in radio inrerface requires 16K in Abis. 8 TS multiply 16k = 128K. So one TRX requires 2 TS in Abis. If you have only 64K on Abis you can have only 4 TS in Air int.
You must have at leas 1 TS in radio interfdace for BCCH, so you have 3 TS in Abis for traffic. If you use Full Rate you have 3 speech channels in these 3 TS in Abis. If you use Half Rate you have 2 speech channels in 1 TS, so in case of 64K you you have 3 speech channels Full Rate or 6 speech channels Half Rate.
In minimal configuration you can only have 32K in Abis for 1 TRX, in this case one 16K in Abis and Air for signalling and 1 for traffic which in case of Half Rate carrys 2 speech channels.