The problem is that it’s not clear whether many/much is referring to “plastics” or “oceans”. Here’s a slightly better way to put it. (By putting “many” immediately after the word it refers to, the number is obvious):
“Plastics, many of which are ingested by fish, flow into oceans.”
While that makes the referent of “many” clear, the order of events is not, so here’s how I would prefer to phrase it:
“Plastics flow into oceans and are subsequently ingested by fish.”
If you don’t like “subsequently”, you could simply put “then”.