This question requires a few assumptions and exclusions:

# Starting with the population of the world (about 7 billion) only North Americans (specifically Canada and the USA) have the Santa Claus mythology. Parallel symbols in other European countries are excluded. Knowledge of "Santa Claus" does not count as belief.

This reduces the number of potential believers to about 300

million. # Adults (people from 10 on up) typically do not believe in a Santa Claus. This discounts merchants who believe in Santa Claus as an advertizing symbol. Based on the CIA database the demographic for people 14 and under in Canada/USA is about 20% (60 million) so the potential population containing believers is 60 million or less.

# Reduce this by 10% to remove those over 10 gets you a residual of 50 million. # Many North Americans do not have an ethnic origin (10%) or religious belief(10%) that believes in, supports, or condones Santa belief. (These numbers are guesses made on national origins and religious affiliation data) This would reduce he number of beleivers by an additional 20% or so to 45 million. # Reducing the number again by folks too poor to support a belief in Xmas gifts from invisible friends will not be done as it is too depressing. So the number of Santa believers is likely around 45 million world wide (0.6% of the world population). This is likely high as the age of figuring out that there is no Santa has been set at 10 but is more likely lower.