I’m using USRTRACK to score the fluence. If I want to calculate the total number of particles in the region I need to multiply by the width of the energy bins, multiply by the volume and then divide by the track length? If so is it possible to find out the track length?

The concept of total number of particles in a region is ill-defined, since it makes no sense to count a particle traversing the region the same as another stopping in the region or another generated inside the region. Therefore, the proper concept is indeed particle tracklength [cm] over the region, which is what USRTRACK gives you already if you do not specify the volume in the card (with no need to multiply by the width of the energy bins, since the integral is already performed in the sum.lis file). After dividing by the region volume (through the card or at post-processing level), you get the average particle fluence over the region [cm^-2]. To get a rough estimate of the number of particles, assuming they all travel the same distance over the region (which may be a poor assumption, see above), one may divide the total tracklength by that distance.

USRBIN is designed to give a spatial distribution of the energy-integrated particle fluence, with 3D bins of your choice over which the particle tracklength is calculated and the average fluence is given as result. There is no energy dependence information out of USRBIN. So the answer is no.
USRTRACK is perfectly fine to get a particle energy spectrum over the region of interest, in terms of differential fluence [GeV^-1 cm^-2] or tracklength [cm/GeV], which is integrated over the energy range in the sum.lis file.
Neither of them counts particles, for the fundamental reason I gave before.
Particle counting can only be performed on a surface (i.e. a boundary between different regions) and is provided, on an energy differential basis, by USRYIELD and USRBDX.

Ok thank you. So I can use USRBDX to calculate the number of particles entering and leaving the surface of the target region, which would give the particle count in the region?

Again: the particle count in the region is a meaningless concept, see the first sentence of my first answer above (adding the case of a particle generated and stopped in the same region). It’s not a matter of how calculating it, it just makes no sense as such.