Estimating the total number of users of illegal or socially-disapproved substances in a given city, state, or country is very difficult. Reporting changes not only by variations in actual usage; people tend to be less willing to report use the older they are, based on current social climates, and a for variety of other reasons unrelated to actual usage. With most statistics on recreational psychoactives, the data can only be used to show trends in reporting and cannot be used to reliably indicate actual usage. However, the numbers from these flawed surveys are all there is to go on.

Overall, it appears that somewhere around 3-10% of people under 40 in the United States, Britain, and Canada report having tried psilocybin mushrooms, with numbers higher for people 20-30 years old.

United States
In the United States, one of the two largest yearly surveys is the National Household Survey which regularly asks about "hallucinogen use" and has included a question about psilocybin mushrooms. According to the 1997 report, an estimated 5% of US residents (10.2 million) had tried psilocybin mushrooms. -- Office of Applied Statistics, 1998, table 78B & Full Report

Canada
In Canada, a 1999 survey of high school students in Ontario found that 13.8% reported having tried "hallucinogens other than LSD or PCP such as psilocybin or mescaline".
-- Hallucinogen use among high school students in Ontario, 1999 (excerpted from Drug Use Among Ontario Students, 1977-1999: Findings From The OSDUS, pp. 111-121)

Scotland
In Scotland, a drug-use survey was conducted in 1996 with 3,200 individuals ranging in age from 16 to 59. This survey included a question specific to psilocybin/mushrooms. Numbers reflect a sharp dropoff in reporting psilocybin mushroom use in respondants over age 40. Below age 40, responses range from 8-25% for males and 2-7.7% for females. The average use (including those over 40) of those surveyed was around 5% in the 1996 survey.
-- Scottish Criminal Justice Research Findings, 1997