Reverse engineering the acceptance rate?
I would suspect that the acceptance rate is calculated against the total number of applicants (so 10 acceptances). But, unless you read the number in some official document that also precisely specifies how exactly this number is to be computed, I don't think you can (i) say for sure what it means, (ii) assume that the number is accurately computed against any particular approach you know, or the person who computed the number knows. For example, if you get the percentages of acceptances from a department web site for the previous 10 years, you have to expect that these ten numbers will have been computed by different people and with different definitions.