Whenever something breaks, I buy a second of that same thing. My max is 1. Not everything is breaking all the time, so it isn't too much stuff. This is for parts that break. I keep a spare pull string of each diameter, fuel caps, trimmer speed feed caps and springs etc. This way when something breaks on-site, I can fix it immediately. If I have a mower or trimmer that breaks down beyond repair, I keep the old carcass around to harvest good parts off of.
Now, consumables is totally different. I track how often I need to replace the filters and plugs. Then, I do all my tools of the same type at the same time. This way they're not breaking down when I need to be using them.