The study enrolled more, but slowly increased the number of vaccinated since the start (which was initially, I'd expect, 0) -- the second dose was still not received by all enrolled as of November 13:
"The Phase 3 clinical trial of BNT162b2 began on July 27 and has enrolled 43,661 participants to date, 41,135 of whom have received a second dose of the vaccine candidate as of November 13, 2020."
So there is no single date one can reasonably use 41,135 or ~20,567 for the efficacy calculations. As the observations continue (expected up to 2 years) there could be much more useful data there.
I also guess the people who haven't received both doses but got sick before are separately counted.
The only useful numbers at the moment we have are 8 and 162 (and 1 and 9 for the severe cases). And that we can assume that from all thousands vaccinated these 170 in total got sick by chance, and that the thousands were carefully enough selected to represent bigger population, and using that we can expect that the efficacy won't too dramatically change as more data come in, the same way as the polls can use smaller samples to conclude something about bigger population, if the sampling is careful enough. But some changes are expected.
It appears it's still too early to know if those who are vaccinated could protect those who aren't, or if the vaccine protects only those who received it (1) -- but anyway we know that still not all of those who will be vaccinated will be protected from having a severe case: there was up to now one severe case in the vaccinated group, versus 9 severe cases in the placebo group, 11%.
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1) https://www.medscape.com/viewarticle/941030
"In an ideal world, a vaccine would prevent infection entirely and, it follows, also prevent disease and severe disease. But this may be hard to achieve for a respiratory virus vaccine. Animal challenge data suggest that vaccinated animals may still be infected even if they don't experience symptoms. A vaccine that is able to reduce the severity of disease, even if it cannot prevent infection entirely, would obviously still have enormous public health value. Therefore, this is what trials target as their primary aim."