I hope you remember your Statistics class where you discussed number of samples and how that told you how sure you were of your answer.
For example you could go to 10 restaurants and if you found 2 then you can write that 20% of the restaurants use this with a margin of error being about 30%. Try 100 restaurants and your margin of error is under 10%
You can find your answer by the end of today with the 10 samples and about a week for the better margin of error.
Then again, you could try google.
How many restaurants use guest paging systems? Any suggestion would be appreciated.