How I understand seed phrase is a secured phrase of my private keys.
And then I have a question:
If I create a wallet I get one seed phrase, one account and one private key. If I push another private key in my wallet then it has two private keys (two accounts) then the seed phase has to change then I can recover my wallet with two accounts (two private keys) if I need.
Do I right understand this moment? The seed phase will change automatically.
If I create a wallet I get one seed phrase, one account and one private key.
Nearly. You get one seed phrase, but that can translate into multiple private keys, and therefore multiple accounts.
If I push another private key in my wallet then it has two private keys (two accounts) then the seed phase has to change then I can recover my wallet with two accounts (two private keys) if I need
No - the seed phrase never changes.
The part you're missing is the idea of HD - hierarchical deterministic - wallets.
An HD wallet starts with a master key, which can be represented by a mnemonic. The master key - and therefore mnemonic - never changes.
From the master key you can then derive child keys, each representing a different account.
Background:
