why 8 slots in TDMA frame of GSM?

Answer 1

It has to do with the bandwidth available per carrier. You don't want too many slots as this requires more bandwidth, thus a weaker field stength using the same amount of RF power and the less spectrum available for multiple cell sites in a geographic area. You use to little slots and each cell site is able to handle less calls/data and you set yourself up for network busy signals for the users. For a detailed explanation of slots, bits per frame, FEC, etc, see this link:
http://www.eecg.toronto.edu/~nazizi/gsm/system/