Hi Jos,
I am not quite sure what you mean by count. Once you have the population sizes you can to anything you want with them. They do come as a quantity with unit ("people"), so if you want just the plain counting numbers you can use:
MapAt[QuantityMagnitude, countriespop1994, {All, 2}]
The countries for which the population size is unknown can be dealt with using:
MapAt[QuantityMagnitude, Select[countriespop1994, NumberQ[QuantityMagnitude[#[[2]]]] & ], {All, 2}]
Once you have done that you should be able to count these numbers, add them up etc.
I am not sure though whether that answers your question.
Cheers,
M.