In its Whitepaper, TON suggest that it is able to support 2^32(about 4.2 billions) of different "cryptocurrencies", "coins", or "tokens", distinguished by a 32-bit currency_id. Each workchain has a basic cryptocurrency, and can have several additional cryptocurrencies.
There is one special cryptocurrency with currency_id = 0, namely, the TON coin. It is the basic cryptocurrency of Workchain Zero. It is also used for transaction fees and validator stakes.
In principle, other workchains may collect transaction fees in other tokens. However, some smart contract for automated conversion of these transaction fees into Grams should be provided.