A cryptocurrency account is a collection of transactions sent to addresses belonging to that account. An account is also a unit of privacy because transactions sent from different accounts in the same wallet never mix, unlike transactions sent from the same account.
As other hierarchical deterministic wallets, Trezor uses account structure to organize transactions. The number of accounts supported by Trezor is not limited, nevertheless in Trezor Wallet, the number of accounts per cryptocurrency is limited to 10.
See also: User manual:Labeling
Standard vs. legacy accounts 
Legacy accounts are available for those cryptocurrencies which have introduced a new form of standard transactions like SegWit. These accounts hold transactions of the previously standard type that have been made obsolete. Although it is recommended for various reasons to use only standard accounts, legacy accounts are valid means for holding funds and making transactions.