It would not be impossible to make an educated guess.

1. Compile a list of major titles from 1936-Present
2. Find production numbers for as many of these as possible. Where not available, make an educated estimate
3. Estimate the percentage of units still in service, giving higher percentage to newer machines

This would at least provide a ballpark figure of the overall pinball population in the world. From there it would be a little tougher to estimate the percentage of those machines owned by private individuals at home.

It would take some homework, but it's not impossible to make a good guess.