Depends on how many companies are doing it in your area. How big of a city you live in, and how far out you’d be willing to drive to get business.
Pretty simple deal though, basically it be just replacing rotten/broken lumber, hammering nails/screws in, and then using a deck cleaning solution that brightens the wood, pressure wash the crap out of it, then spray a sealer on it a few hours later.
You might make decent money at it in the spring/summer if you live in a city with a large population, with decks.
but then again it might take a few years to get a name out there unless you're just wanting to do 1 or 2 decks on weekends