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

It's kind of up to you! You don't have to eat a certain 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. 

Unless you have a great appetite 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.