A 10-speed mountain bike can still work fine if you're on a tight budget or just getting into the sport. You'll typically find these on entry-level and older models, and they often come with two or three chainrings up front (called 2x or 3x setups) rather than the single chainring that's now standard on modern 12-speed bikes.
The trade-offs are real, though. You'll get a narrower overall gear range, meaning fewer options for steep climbs and fast descents. The front derailleur adds weight and shifting complexity, and the budget-level components on most 10-speed bikes tend to be less durable over time.
Also, don't count on upgrading later—moving from 10-speed to 12-speed isn't a simple swap. It usually requires replacing the entire drivetrain, rear wheel hub, and sometimes more. If you're riding milder trails casually, a 10-speed will get you out there. Just go in knowing that upgrade paths are limited.