This is the 13896th most frequent Korean word.
많으면
"If there is a lot" or "If abundant."
Here, '많으면' is used to denote a conditional statement about a large number of people.
사람이 많으면 자리가 부족할 수도 있어요.
If there are many people, there might not be enough seats.
In this sentence, '많으면' refers to having a large amount of time, again in a conditional context.
시간이 많으면 여러 가지 일을 할 수 있습니다.
If you have a lot of time, you can do various tasks.
Here, '많으면' is being used conditionally to indicate an increase in air quantity.