for a 20 gallon, i think 2 bottles is good. So you should have 4 bottles handy, 2 bottles attached, and 2 bottles ready with the new mixture.
But yes, you can reuse the bottles, just rinse with water till its clean and remake the mixture. It takes about 30 minutes to 2 hours before it starts producing co2, which is why you should have 4 bottles handy, so when you are about to switch them off, you can make the mixture early in the 2 spare bottles =)
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