As magma is brought up by volcanoes and other forms of crustal creation, other parts of the crust are being pushed deeper into the mantle, where they melt and become magma again. In doing so, the magma renews itself. The Earth will not run out of magma until it has lost sufficient heat that the crust no longer melts.

What happens if we run out of magma?

There, it gets recycled, becoming part of the mantle. When a volcanic explosion is so massive that the magma underneath is completely depleted, it forms a caldera, or bowl-like depression.

Does the earth have an unlimited supply of magma?

The magma supply of the Earth is not unlimited, but instead it has a fairly constant volume.