First of all, the reactions are not self-sustaining or anything, they require constant intervention to be carried out.

Most unavoidably though, the global energy use last year was about 10^20 Joules. This is approximately equivalent to 1000kg or 1 tonne, obviously there is an efficiency in there somewhere but that means that you need to turn 1 tonne of matter into energy per year to fuel the entire world. The Earth weighs 10^21 tonnes and although we can only fuse specific elements (mainly hydrogen) there is still such an abundance of that in the sea alone that we will not run out for millions of years even if our energy consumption went up a thousand fold.