How do I make it so there's a finite amount of blocks I can place?

So my game involves placing blocks to guide characters to the goal, but there's no limit on how many blocks can be placed at once, which makes the game a lot easier. I want to make it so you start with a single block, and can have the characters you guide collect more for you to use.

The finer details depend on the actual gameplay but in general use a global variable to track number of blocks. You can make it subtract 1 when you place a block for example, then when it hits 0 you can no longer place blocks.