Is the given language decidable for turing machine? I am going through undecidability of TM and found this question

$L=\left \{ \left \langle M \right \rangle |M\ is\ TM \ and \ number\ of\ strings\ in\ the\ language\ \ is\ prime\right \}$

I think it is decidable as number of strings in the language is prime ,i.e , 2,3,5,7,11,13,17,............. It seems to me like it is accepting all the strings.

If it has accepted 17 strings, then it must have accepted 12 strings, 14 strings, 16 strings and so on. Actually i am not getting from where to start.

Can anyone provide legal explaination of what is actually happening here ?
Consider a problem 'A' which is undecidable. If 'A' is reducible to another problem 'B', then problem 'B' is undecidable.

Consider following two statements:

p: Turing machine halting problem is undecidable.
s: A given Turing Machine 'M' reaches a state 'q' on reading input 'w'.


If we can modify statement 'p' in such a way that the answer to 's' will be answer to 'p', then we can say problem 's' ( statement 's' ) is undecidable.

Also note that I will be using a term "Dead Configuration" to mean the Turing machine is in a state from which no further transitions is defined i.e, a Turing machine halts because of a dead configuration.

Modified version of 'p': A Turing machine halts only at state 'q'.


That is, whenever a Turing machine reaches a dead configuration, we will make a transition to 'q'.
Modified version pf 'p' is undecidable, which means it is undecidable that a Turing machine reaches a state 'q' on reading any input.
Hence, problem 's' is also undecidable.

On the same lines statement 's' can be modified to prove that the problem, "Turing machine will accept a string" is undecidable. 

Hence, if the problem: "whether a string is accepted by a Turing machine" is undecidable then the problem of counting the number strings in a language will also be undecidable.