15 of them.
27
More than one
6
Possible solutions - using your rules are:- 11,13,17,31,33,37,71,73 &77
There are 60480 numbers.
There are 4^6 = 4096 such numbers.
8.
If the digits are all different then 18. Otherwise, 192.
There are 900 of them.
15 of them.
27
48 of them if digits may not be repeated. 100 if they can.
More than one
6
Possible solutions - using your rules are:- 11,13,17,31,33,37,71,73 &77
There are 2000 such numbers.