1. How can I know how many accounts I have ever created - funded? For example, I know that I have 2 accounts, but what if I actually had a 3rd one and I have forgotten about it? What if I gave my seed phrase to my child and told them to recover it? How could they know how many accounts have addresses that have been funded?
You have to let them know about it. You can use practical example.
2. Do you know any way to "note" or "backup" safely how many accounts have been created using my recovery phrase?
It is you that should remember it.
The only wallet that I noticed this is the non recommendable Coinomi which is close source. Most wallets do not allow it as they will allow you to create/use another seed phrase instead which is recommended.
I prefer to use another seed phrase instead.
If you want to use the same seed phrase, use passphrase to extend the seed phrase to generate you another different wallet (keys and addresses) entirely. If you lose your passphrase, it is also like you lose your seed phrase because you will need both (seed phrase and passphrase) to access your coins.
