How many card readers your business needs will ultimately vary depending on your operational and deployment needs. It is common to use one reader for each instance of your point of sale application.
We also recommend keeping at least one backup reader available per location so that you can continue processing payments from customers in case there is ever a problem with your reader.
A card reader can only connect to one instance of the Stripe Terminal SDK at a time. For example, if you are using a smart card reader and have multiple checkout stations in a store, you will need a reader at each station.