Number of IEEE 754 doubles between two adjacent single-precision floats
Let's consider two made-up number systems: 2-dec and 3-dec. In 2-dec, numbers are specified to two decimal points (in decimal). In 3-dec, it's three. Between, say, 1.2 and 1.3, which are adjacent in 2-dec, we have 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29 in 3-dec.

Hope this helps. 

As an aside, the answer might potentially depend on whether the numbers are denormalized or not.