To add to what the others said there is also a phrasal verb in there. the definition is below. Come up = to be mentioned or talked about in conversation: Examples: What points came up at the meeting? The subject of payment didn't come up. Did any new points come up when you spoke to him? If the subject of divorce comes up, try to change the subject.

Hi Nima, Without knowing the full context of the article, it seems that "the subject of the future came up quite a bit" means: "during our conversation, we talked about the future many times" or "the future was a frequent topic in our conversation". Does that make sense? Best, Kevin

They talked many times about their future (plans?). "Quite a bit" means often or many times