shuf -n 20 all_addresses_with_balances.txt
Would give you 20 random lines from the file with all addresses.
I used shuff before, and it takes 10 seconds reading from RAM. That's too much to update once a minute.
I'll try something else though: every day (or hour), I get 100,000 random addresses. From that subset, I get 20 addresses each minute. This will use significantly less resources.