My wallet on Android showing green addresses 0 to 20 was imported master public key from electrum wallet created in tails.
You should see the same number of addresses on the desktop version of your wallet, unless that wallet is offline and electrum doesn't know that the first address has been used.
Assuming you have used the first address and not more, you should see addresses 0-20 on your online wallet and addresses 0-19 on your offline wallet.
Yes is just the first address used. Have to go online tomorrow and see if that missing address shows on windows version.
Am I correct in thinking that more addresses are not created. Just the fixed amount generated by the seed and the change addresses. What happened when all addresses used up, do old ones get reused again?