How many meals should I have per day to gain weight?

There isn't a right or wrong answer here, and you don't have to eat a specific amount of meals per day. When it comes to weight gain, the most important thing is your total daily intake, both with what you eat, and how much. 

If your appetite isn't great though, you'll likely want to eat at least a few meals per day, and may even want to throw some snacks in there. If you feel full easily, eating more frequently (even in smaller portions) can help bump up your calories.