Prostitution is hard to measure for a variety of reasons, the initial problem being deciding how to collect data.  The authorities may base their figures on arrests or prosecutions (which are dependent on law enforcement priorities and practices which vary from city to city and country to country), whilst NGOs working with sex workers may provide completely different estimates.  Both sets of figures may well ignore forced/trafficked prostitutes, as would numbers of sex workers in a union/organisation.

Differing legislative and policing approaches to prostitution, the diversity of the individuals involved, the complications of women trafficked and coerced into compared to voluntarily in prostitution and the compounding factor of the use of the internet as a tool for soliciting prostitutes and trafficking women mean it is particularly difficult to accurately measure. I maintain that a good indicator as to how much of a problem a government perceives an issue to be is generally the amount of money that government spends on it. In Britain, the Home Office spent 850,000 pounds on reducing crime related to prostitution in 2000. (This figure may be more representative of procuring, exploitation/living off immoral earnings and crime related to pimping such as drug dealing: in 1999 the Home Office produced a report on the links between the sex and drug industries .pdf)

In some Northern European countries the ratio of prostitutes per 1,000 population is between 1and 2, though in neighboring countries the level can be a tenth of that. This variance can depend on population distribution (large cities typically have more prostitutes) and the local nature of demand and social acceptability of prostitution, though action being taken by law enforcers in any given moment in time also plays a role, particularly where the country in question is small and has land borders (as with Luxembourg).

In Holland and Sweden and Germany, it appears that 50% of prostitutes are local, the remainder being migrants, many of whom move between European countries. In Italy, the figure is reported to be substantially higher, whilst in the UK (outside of London) migrants allegedly make up a minority of sex industry workers. 

The figures below are primarily from the EUROPAP regional reports, though the table was compiled for this website.  It shows the current (and changing) estimates of numbers of prostitutes in the EU member States.  For more information on a country, click in the 'Country' column.