Even that has a (still unsatisfactory) answer.
Poincaré symmetry imposes constraints on the kinds of fields we can have. Gauge symmetry shows us how they may couple.
There are still some arbitrary selections of the possible permutations that nature has “picked”.
It would be much more satisfying (not that nature exists to be satisfying) if we could explain our universe starting from some universal constraints on things that must be true of any non-random mechanistic universe, plus some set of (< N) non-forced "it must be A or B" additional constraints, then be able to derive everything known about our universe - fields and symmetries etc - (& ideally predict something unknown) as resulting from some particular selection of those additional constraints.
This seems about as close as we could get to explaining our universe... Basically saying that god flipped a coin marked A and B, and it come down A so here we are. Maybe god kept on flipping sets of coins and created a whole bunch of other universes too, whose physics we could also derive.... and maybe one day visit and confirm.
I'd also observe that between dark matter and dark energy, there's good reason to believe that we may not have a full accounting of all fields.
I am just observing that if you have a non-scientist asking the question "how many fundamental particles are there", with the expectation that "995.5" is not really the right answer, "the number of fields" is a reasonable response that probably gets closer to what they are looking for. Even if someday someone does get them to all be some manifestations of a single field it would arguably still be the case that people are more interested in the answer of the current number of fields then being told "1", because "1" is in many ways not a helpful answer to "how many types of things are there". Even if there is a profound sense in which it was true, there would still be a profound sense in which it was false, too.
It's inductive and abductive reasoning. The one field, and it has lot of mathematical characteristics which makes it unique on its own, and also it is the only one that has a chance to fit, is the e8 field popularized by Garrett Lisi.
If a universe were to be designed based using the e8 Lie algebra as an elemental field, it would look a lot like our universe.
Currently the standard model is a patchwork of field added as experiments for observing particles were possible to realize. The big picture's view is a unified theory which fits perfectly all existing data.
Currently, we don't have any theory that works that's any simpler than the SM. So that's the theory that Occam's razor currently tells us must be true, as it's the simplest alternative that actually works.
It seems there has to be a reason WHY there are exactly N fields, and WHY they interact in the ways they do.
Edit: As I noted in another comment, the best explanation may come down to "there are only 100 viable types of universe, and ours is type 42". I'd be happy with that.
If you look at histogram plots of protons, neutrons, and stability, it's not a perfectly idealized form. It's a rocky plot. This emerges from the quantized nature of reality.
So a periodic table of particles (fields) that looks kind of weird and ad-hoc to us is the expected result.
What we don't yet fully understand is really two things as far as I know. First, we know less about why these particular values are special. For the periodic table we actually understand this pretty well. Second, we do not know if there are other islands of stability or particles-fields we cannot see (e.g. WIMPS). For the periodic table we are pretty sure there are no large islands of stability at higher weights. Not 100% sure, but if they do exist there's probably only a few exotic mega-atoms that could be stable, not many.