Simple, it is a combination of guess and actual estimate. For example the guestimate is that in UK there are around 5200 conversions to Islam every year. For Germany and France the figures are around 4000. People say that gloating over these things makes it difficult for those who want to take the leap, the reason being the distrust for the converts for going over to the other side. One point that should not leave the sight is that some people prefer the word revert over convert. Reversion is a home coming. Those who have faith at least they should know better that every human being is born a Muslim. It is obvious that a person at the time of birth stands completely submitted to the Wish of Lord Most High. Thus a person who is coming back to Islam is rectifying an accident in his (or her) life that took him out of the fold of Islam. If a person is taking a step to revert then that should restore trust in him and not the other way round.
Source for Figures: The Independent UKvia Sunni Forum