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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?
Initially, Electrum generates the first 20 addresses and another 10 that are labeled and used as "change addresses". There is no real difference between receiving and change addresses, it is just the way Electrum uses them, and all funds received at any of those addresses will be added to the balance of your wallet. When those first 20 addresses are used, Electrum generates the next batch of 20 addresses, and so on. There is no limit because the number of possible addresses generated from one seed is unlimited.