I was reading the book "1100 Words", and for the section "Week 19, Day 1" I read the following: (eg https://lelb.net/1100-words-week-19-day-1/)
the number of the king’s opponents who were incarcerated* were legion
then I started to read exercises.
"the number of my followers is legion," said the flamboyant politician.
In the first example, we read "were" which is plural and the second example shows the same in the singular.
Which is correct and what's the difference if any?