What is considered rare is, fortunately or unfortunately, subjective. When it comes to most fandoms I consider any pairing with under 100 fics on A03 to be rare. However, the MCU is such a massive fandom that you'd have to move the goal posts. I agree with novembermond, if the pairing is within the top ten for a fandom then it is not rare, however, I know it can get muddy when some pairings are always side pairings in fics.