The term "selectee" is somewhat clumsy.
If you enter the lottery you are the principal applicant of your ticket, you own it. On the ticket you also list the names of your children (unmarried, under age of 21,?) and the name of your spouse, if existent at that time. These are your derivatives.
If the ticket gets drawn it becomes a case and the names on it become selectees. The Department of State keeps a master list of these names and speak of registered applicants. The number of registered applicants grows in time because principals could add a newborn to the case, or a single principal marries her/his loved one and adds the spouse to the case.
So the 83,910 (published) selectees are a first snapshot of the registered applicants at the time of the draw.
The principals compete for the available diversity visas. If a case is approved the principal applicant and each derivative (who passed background checks) get a visa. So a case can get several visas.
The number of issued diversity visas cannot exceed 55,000.