There is a very useful surname-population online database, created originally from the statistics held by the Office of National Statistics and extracted as part of the Llangynfelyn Digital History Project, which allows estimates to be made of the population of a given surname in England and Wales. The website points out that the database is not totally up-to-date nor error-free, but that "experience suggests that multiplying the result for your surname by 0.93 will give a good idea of the living population for your surname, and multiplying by 3.5 will give the population since the start of parish registers in the 16th century."
On this basis, we can derive population statistics for the three name variants that it lists, as follows:
|Name||DB Pop||DB ranking||Living population||Pop since C16th|
Unfortunately, comparable data is not available for Scotland, since the surname surveys done there are focused primarily on the 50 or 100 most common surnames in use - and Shankland is far from being in this list!