Q1: Yes. Usually.
Q2: If on several occasions a large amount of money was spent, then we can say "large amounts were spent".
Q3: Same as Q2. To illustrate:
1 March 2003 - 23 vehicles towed
2 March 2003 - 40 '' ''
3 March 2003 - 25 '' ''
"In April we decided to install a gate, because large numbers of vehicles had been towed in March."
However, we can also consider the March total as a whole (say 850 cars towed). Then we would say "... a large number of vehicles had been towed ...".
It is often possible to treat a group either as a single entity or as a number of constituent parts, depending on the context. For instance:
"The council made its decision." (acting as one)
"The council had their coffee after the meeting." (each member did so individually)