I believe this question is more related to bitcoin technical aspects and I am asking this question in this section rather than in the section dedicated to hardware wallets.
Mnemonic algorithms like BIP-39 are essentially a Key Derivation Function[1] themselves (BIP39 uses PBKDF2 under the hood as well) and KDFs don't really have a limit on how many keys they can generate. You should be able to generate an unlimited number of them as long as you can change the inputs of the KDF and in case of BIP-39, you can do that since it takes 2 inputs: the mnemonic and the passphrase (the extended words).
On top of that you have the derivation path which has some limitations like the depth but by changing the index and the path itself you can generate virtually unlimited number of keys. That is: m/0, m/1, m/2, ... m/0/0, m/0/1, ..., m/0/2, ..., m/1/0, m/1/1, m/1/2, ..., m/0/0/0, m/0/0/1, ...
[1]
https://en.wikipedia.org/wiki/Key_derivation_function