The statistics show around 300 FI ratings issued per year. Assuming a flying career of 25 years that means there are around 7500 holders of FI ratings. Many will be dormant, but it is probably a reasonable estimate that there will be between 2000 and 3000 valid FI ratings.
It is interesting to note there has been an increase in the number of FI ratings issued in the last 3 years. It is not possible to accurately determine the number of ME instructors as they are combined with CRIs and TRIs. Probably quite a low percentage.