Level 50: By 1820, there were over sixty steamboats on the Mississippi river, ………. were quite luxurious.

Knowledge: Relative clauses

Explain:

“which” replaces nouns of things; plays the role of subject/object in a relative clause

sixty steamboats: 60 steamboats

Rule: quantity phrase before “which” => many of which

By 1820, there were more than 60 steamboats on the Mississippi River, many of them quite luxurious.