For example: How many petrol stations are there in the UK?
Surely the quickest way is:
1) Make an assumption of how many stations serve your home city's population (say 10 for 300,000 people)
2) That means 1 station serves around 30,000 people.
3) Get the total population of the UK approx. 60m, divide by 30,000 = 2000
4) Then adjust for areas with more service stations London, Birmingham etc