The Pfizer vaccine is purported to be 90% effective. However, in other vaccines, the effectiveness depends upon the age of the population vaccinated, so I would expect this new vaccine to be similar. In older, more vulnerable, populations, the vaccine is likely not as effective.
In Britain, the best the NHS can manage is to inoculate about 75% of at-risk people against influenza. In such people the vaccine is about 75% effective. So that gives only about 56% coverage (0.75 x 0.75). They would do even worse for those who are not at-risk. Achieving herd immunity through vaccination to Covid-19 would therefore be difficult, as it is thought that you need 60-80% coverage of the whole population, not just those who are at-risk. Natural immunity might currently be at less than 10% of population at the moment.
This is a two-dose vaccine, as well. So divide the dose figures by two to get the number of people who can be vaccinated by any amount of vaccine.