My wallet on Android showing green addresses 0 to 20 was imported master public key from electrum wallet created in tails. I could give a screen shot but don't want to reveal by used public addresses.
The default gap limit on electrum is 20. This means that electrum generates more addresses until in reaches 20 consecutive unused addresses.
Your wallet is showing 21 addresses is because you have used the first address. That should be the same on the desktop version of your wallet, unless that wallet is offline and electrum doesn't have any information about your first address being used.