A foot is a term to describe a unit of meter, and can vary in its exact definition, depending on the type of poem. It's based on the count of stressed and unstressed syllables. The confusing thing is that there are different types of feet, containing different numbers of stressed/unstressed syllables. However, every type of foot only includes one stressed syllable per foot. To count the number of feet in a line, count the number of stressed syllables:
In the line:
Round about Shalott.
The stressed syllables are in bold, they're the syllables that you put emphasis on. There are three, so there are three feet in line 9
Answer is 3