I think the only factor that can help in answering is the number of address. The total number of addresses, of course, gives an upper limit for the number of possible bitcoin users although not an exact estimation since everyone has multiple addresses of course and plenty not even re-using an old address and make a new one.
Having the upper limit of the possible Bitcoin users wouldn't help at all since it would be really really far from the actual number of people using Bitcoins. The upper limit becomes irrelevant. It's just like when we say that we the lower limit is 0 and the actual number is in trillions.
Not only that people are using multiple addresses but also there are websites like gambling sites, exchange sites, and other bitcoin wallet sites that has thousands of addresses for their current and future users.