You can calculate the average number of simultaneous TBFs per PDCH by using the ratios: TBFPDLGPRS/PDCHDLGPRS and TBFPULGPRS/PDCHULGPRS respectively for DL and UL. These counters are found under the object type TRAFFGPRS.
If the number of users per PDCH is high, then the throughput per user will be low. You should consider adding more PDCHs if the above ratios are high.
Yes, the number of users per PDCH is 16 for DL and 7 for UL.
You would experience many retransmissions when there is interference(as a result of a poor frequency plan/allocation). However, a high retransmission rate can also be as a result of other things, eg faulty TRX, using a static coding scheme, etc.