One or two, carefully placed as ornaments, are fine. Say a nice one per five gallons.
Most gravels also leach carbonate into the water, they just do so more slowly than shells and crushed coral. BUT keep in mind that if you're using artificial driftwood in the tank, shells are a good way to counteract the softening and pH drop that driftwood causes. (Natural driftwood doesn't pull as much carbonate out of the water as the artificial acid produced driftwood)
Do observe for sharp edges, Betta are thinking fish who will often try to hook themselves to the bottom to sleep. I can't keep small plants down in my girls' tank because of it. Remember to examine any ornament you use for this issue.
For about a year I used my first male's disco ball bowl as an ornamental bowl in his tank to deal with the flow from the filter. I could simply siphon out from the little bowl since he spent all his resting time in it.