This article says that 5 to 13 million metric tons of plastic go into the ocean every year. In the study At what point do floating plastics sink?, they give an estimate of the weight needed to sink plastics. They reference the paper Assessing marine debris in deep seafloor habitats off California, which assesses the amount of plastic debris that has settled to the bottom of the ocean off the coast of California. Given this information as a starting point... How much plastic settles to the bottom of the oceans each year?
About 18.2 million tons sink to the ocean floor.
Every year, an astonishing 260 million tons of plastic waste is produced, and about 10% of it ends up in the ocean. Over time, roughly 70% of the plastic that ends up in the ocean sinks to the ocean floor.
Therefore 18.2 million tons of plastic waste end up on the ocean floor.
I used this article for my numbers.