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You get 1 seed phrase (although you can always erase the Trezor and put another seed phrase on there).
Your 1 seed phrase will be „converted“ into the private keys for whatever coins you have. Even though each blockchain has a different format of private key, the Trezor knows how to convert the same seed phrase into the proper format of private keys (and it will do it the same way every time, so if you get a new Trezor and put the same seed phrase on it, it will convert that to the same keys for the same coins again).
This conversion is not a unique „Trezor“ feature. You can put that same seed phrase into any hardware or software wallet that supports the BIP-39 standard and it will also then have access to your coins (but please do not actually put your seed phrase into a computer, even though it would work, unless absolutely necessary, because that could compromise your seed phrase).
Also, you can enable „passphrase“ in Trezor, which adds another word of your choosing to your seed phrase (making it easy to have multiple wallets on the same trezor). You have the same 12/24 base seed phrase PLUS your additional passphrase(s), which gives you access to unlimited wallets.