Question

how many decimals are between 0,5 and 0,6 ?

Answer 1

Between 0.5 and 0.6 can be an infinity.

ex:

0.51

0.52

0.517

0.5169889549.......

0.5999999999..........

etc.

Answer 2

Infinite numbers are between every decimal number. 
Infinite decimal numbers are between every whole number.