We keep common torsion spring sizes, standard rollers, and cable on the truck, along with the opener parts that fail most - drive gears, belts, logic boards for the usual residential brands. For a door that's close to the 1987 median build year for this part of town, that covers the large majority of what breaks.
The honest limitation: if your door is an older one-piece style, or an opener model that's been out of production for years, the exact part sometimes isn't something we carry stock of. We can usually source it, but that can mean a return trip instead of a same-visit fix. We'd rather tell you that up front than have you wait at home for a part that isn't coming today.
We don't stock every spring wire size and every drum combination for every door ever built - nobody does. What we carry covers the hardware that actually shows up in houses from this era, which is most of what we see out this way.