when speaking of isolation mechanism, we make a difference between pre and post-mating. So where does "different number of chromosomes in parents" belong to. I can imagine a zygote not being able to form because two species have a different number of chromosomes. But I can also imagine a zygote being made - in a case of mule - it's parents have different number of chromosomes.
Thank you!

Actually termininology depends a bit on the taxon, so - for instance -people tend to talk abouty pre- vs post copulatory mechanisms of isolation. This makes it clearer where the dividing line is. Chomosomal incompatability of the sort you describe is post-copulatory since either the egg is not fertilised, or the egg is fertilised but the embryo not viable, or egg is fertilised and embry is viable but produced an infertile offspring. Regardless this is subsequent to the two parents copulating. In organisms with external fertilisation (many fish, plants etc) we often refer to pre- vs post fertilisation mechanisms (so a slightly different, buty nonethless clearly defined dividing line).

Agreed and one additional point to emphasise - failure of fertilisation and/or development when sperm and ova are mixed from different species is NOT just because of differencing number of chromosomes between the two species.

There will be a myriad of proteins and immune processes that are for want of a better word "incompatible" betweeen the species that will stop fertilisation and/or development occurring.