Numbers are a dangerous thing. They help us measure things, but measuring in the spiritual realm isn't always a healthy or holy thing. In one sense, we count people because people count. I get that. But there is a great danger when it comes to churches. Numbers tend to produce pride or jealousy. And both of those things will eat you alive spiritually.
I find it interesting that counting numbers was one of David's downfalls. In II Samuel 24 he tells Joab to number the people. Joab asks an insightful question: "Why does my lord the king delight in this thing?"
David wasn't just counting. He was delighting in it. But I don't think it was always that way. When the people sang about David killing his tens of thousands it didn't translate into pride, but something changed over time. David is relying on numbers. David is delighting in numbers. It's a prideful numbering. David was conscience-stricken after he took the census, but it was too late. The punishment for David adding up? God subtracted 70,000.
The point? Be careful how you count. And if it results in pride or jealousy, stop counting. The Lord will not share his glory!