there need only be one, hazzy.....and that would be the one you personally choose to use/model in.
However,like all things, model railways is an old, very broad hobby.
so like all things, it covers all tastes and needs.
Since it involves MODELS.....even if considered as toys....then these models, which are supposed to [more or less] represent the real thing....need to be made to a SCALE.
The scale of the model gives a sense of proportion.
Even if making a''wooden toy train'' for a child, one attempts to keep a sense of proportion about the thing, otherwise it ceases to be recogniseable as a train to the child.
''Action Man'' and Barbie are made to a scale, to provide proportion......any old head stuck to any old representation of a body simply ''won't do?''
so, as with all toys and models, model railways come in a variety of different scales....
some arose out of a recognition that modern families tend to live in match boxes these days, so space is at a premium.
Hence, the likes of N gauge was developed, to fit more into less space.....because it uses a smaller scale, than say, OO or HO.
the opposite applies to O gauge....
[[scale and gauge are two entirely separate things.....quantities if you like.....and the relationship between scale and gauge is complicated, in model as well as prototype.....and the differences are of a historical nature....and stemmed from engineering issues.
[scale is the proportion, or ratio of the model to the real thing........gauge is simply the distance apart the rails are]
from your other post, you need only concern yourself with a scale of 4mm to the foot....and OO gauge track