How much do the stores keep from my sales?

It varies from store-to-store. With downloads, you will be paid the wholesale price attached to your track by the particular store. Stores generally take about 20-30% of the retail price after sales tax.

With streaming, when a subscriber to a store streams your music you get paid a proportionate share of the subscription revenue (or, if the service is ad funded like Spotify’s free tier, the ad-revenue) collected by the store in that particular month. This means that payments will vary from month-to-month depending on the amount of subscription or ad revenue that was generated by the store and how often your music was streamed. 

For more information on how much Spotify and other streaming services pay artists, please visit this article.