You could always ask Referee magazine. In the past they have published numbers of officials in various sports in the US.
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If you care about Europe, we use NCAA Rules, rough numbers that may be a couple of years out of date......
Austria 50
Belgium 20
Czech 20
Denmark 200
Finland 150
France 300
Germany 880
Ireland 10
Italy 100
Netherlands 40
Norway 50
Poland 15
Russia 10
Spain 50
Sweden 250
Switzerland 50
United Kingdom 100
However what you have to understand about officiating in Europe is that for many countries the only way they get enough officials to cover games is to force teams to to supply 2 or 3 guys each year to officiate.
So the figure of 250 for Sweden for example sounds high, in reality the number of neutral officials who are not press-ganged team members is probably less than 50.
UK does not currently use this type of system (but may in the near future) and has about 100 officials of whom maybe 70 working regularly.