This is going beyond my expertise, but I'll give an attempt at an answer. Take this with a massive grain of salt. It's almost certainly some level of wrong:

The trapped light does not take up physical space in the same way that matter does and can lead to an increase in the local energy density within the photonic crystal. However, it is not clear how this energy could be efficiently extracted and used for practical applications. Also, the confined light is subject to energy loss through various mechanisms, such as absorption by the material or scattering and these losses would limit the amount of energy that can be effectively stored in the system over time.

They're mass-less, and are usually thought of as a zero-dimensional object, having no volume and taking up no space.

You can theoretically have an unlimited number of them in a space due to this, but due to other properties of bosons (like photons), there are other limits. In particular, if you accumulate enough of them they spontaneously spawn new particles!

They aren't really physical particles in the classical sense, they cross quantum fields, and to the extent that those occupy space, the photon isn't in any one position, but merely crossing it or spread out across it.