Looks like 11 days in Sept 1752 when England adopted it. But earlier in 1582, 10 days were eliminated when the Roman Catholic countries adopted it. Looks like the answer depends on the country or countries referred to as "changing their calendar."
Wednesday 2 September 1752 was followed by Thursday September 14 1752, so if your birthday was on one of the "missing" dates then hard luck - no birthday presents for you in 1752.