Because words from the passphrase list can be repeated and put in any order, this means there are an exponential number of potential passphrases.
Each word in your passphrase can be any word in the 2048 words, so the number of possible combinations would be 2048¹² ≈ 10³⁹ and the phrase would have around 128 bits of security. This is approximately the same strength as all Bitcoin private keys, so most experts consider it to be sufficiently secure.