Hi Georgiegirl2012, et al,
with any product or consumable item, I think it is important to have access to a representative sample of the population. Only by doing this can you make informed opinions as to the value of that product.
I'm not sure what a statistically representative number would be, but it must logically be greater than one. And given that the products on the market very rarely share identical capabilities and features, then you owe it to your peace of mind to be certain that you have at least one that is "right."
Given that no product lasts forever, you may want to invest in a degree of insurance to protect your reading activities. This means, of course, more than one reader.
I'd suggest you now have some perfectly good reasons for acquiring sufficient readers for you purposes.
Regards,
Michael P