The department sometimes ‘groups’ questions for answer because they are on similar subjects. This means they may be taken out of numerical order.
In this case, the MP whose question comes first in the group will say “Number X, Mr Speaker”, then ask their supplementary question, and then the other MPs whose questions have been grouped with it will be called to ask their supplementary questions.
You can see which questions have been grouped each day on the annunciators (the monitors around the parliamentary estate) before the start of Question Time.