Minimum Sample Size Required

I am a total novice...I do hope you can help!

100,000 people usually shop in Store X.
But, only 50,000 people shopped in Store X in Period Y.
10,000 people (of the 50,000) purchased Product Y.
Purchase Rate = 20%.

How confident am I in the results?

And, what minimum sample size did I need in order to be 95% confident in the results?

I've gotten myself into all sorts of difficulties - sometime a little knowledge can be worse than being a novice!

Thanks for your help in advance.
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