How does number of transactions in the block affect it's mining time? Does number of transactions in the block affect it's mining time? 

How and why?
The number of transaction in a block negligibly effects the mining time. The only effect is that more computations need to be done to calculate the merkle root during block construction, but that is a very small fraction of the number of calculations needed to be done during mining itself. Mining only hashes a fixed amount of data so including more or less transactions does not really make a difference.