That depends on what’s important to you, of course?
On my 29″, I run a 32T front, with the big X01 cassette on the back, it gives me a bigger range than my previous 1×10 setup, which I had been running for probably 3 years & never ran out of gears riding up, or down.
That equates to something like ~30mph in bottom gear spinning at a good rate. If you want to ride down stuff faster than that, whilst still pedaling hard, I would suggest you probably find something more interesting to ride down. If you’re riding trails that fast and still find you haven’t got enough gears, you should probably be entering some races….