Your island can have a maximum of 6 rocks in Animal Crossing: New Horizons. If you have fewer than that, then a new rock will spawn in a random location on your island every day until you have 6 again. Having more Animal Crossing Bells
can help you enjoy the game better.
If you are instead asking the crafting material stone, you can get stone by hitting rocks. Each rock could be hit up to 8 times every day with an ax or maybe a shovel, provided you hit them fast enough. Also, you should haven’t eaten fruit before hitting them, or else you will destroy the rock. If you have eaten fruit, you may sit on a toilet-related item to eliminate the fruits out of your system.
If you've exhausted the rocks on your island, you can visit other islands with Nook Miles Tickets
to find more rocks. You can also get stones randomly through fishing, but since it isn't guaranteed, this is an inefficient method to get stones.