# a hundred times greater than it is

Joab expresses the desire for an army the size of 100 armies to say he would like the army to have more soldiers and be more powerful. 

# But my master the king, do they not all serve my master? Why does my master want this? Why bring guilt on Israel?

"But my master the king, they all serve you already. My master should not request this. You will only bring guilt on the people of Israel by trusting your military power." 

# Why does my master want this?

The word "this" refers to David's plan to count all the men of Israel.