For ponds, a single kit is usually enough as long as subsamples have been merged from around the perimeter of the pond.
As the size of the waterbody increases, so should your sampling effort.
The precise number of samples will usually be a trade-off between your budget and the amount of spatial resolution you need or the importance of detecting rare species.
For most lakes, 5-10 samples are sufficient, given appropriate merging of subsamples but you may want to take more if you are targeting rare species. In rivers and streams, it depends on the size and flow rate of the waterbody and the area that you need to survey.
We are always happy to advise on sampling design for specific projects.