Two mainline Kirby games were released on the Nintendo 3DS: Kirby: Triple Deluxe and Kirby: Planet Robobot. They are the two original 3DS entries generally counted in the series’ mainline platforming sequence.
Kirby: Triple Deluxe was released in 2014 and became the tenth main installment in the Kirby series. Its stages used the 3DS’s stereoscopic 3D effect and allowed Kirby to move between foreground and background planes. The game also introduced the Hypernova ability.
Kirby: Planet Robobot followed in 2016 as the eleventh main installment. It retained the layered stage design of Triple Deluxe while adding the Robobot Armor, which could scan abilities and transform to interact with obstacles and enemies.
The number can seem higher if spin-offs, enhanced ports, and downloadable games are included. The 3DS also received Kirby’s Extra Epic Yarn, Kirby Battle Royale, Kirby’s Blowout Blast, and other related releases, but those are not counted as the two mainline entries in this question.