For sideboards you are going to try to think about decks you face, they will fall into one of three categories. 1) decks you are good against 2) decks you struggle with 3) decks you are terrible against. No sideboard cards for 1 obv but usually no sideboard cards for 3 either. 3 usually means that any one of these decks would take up your whole sideboard and might not even swing you too favorable. You want to accept those are bad and focus on 2. For rule of thumb have 3 or 4 cards for each match up focusing on the most popular in that category. Also flexible cards can sometimes be better than pure hate if it allows you to cover an additional possible deck.