First and foremost, you should give your body ample time to put on that extra 18-20# you are desiring, especially if you want the mass to be
lean body mass and not all fat mass
Second, putting on that kind of mass
does require you to eat and eat and eat, not only many calories per day but several
times throughout each day as well. Having yourself on a feeding schedule can make a big difference in your success level with this.
Third, going back to the first point I said here, you want to be sure to keep your diet (though BIG AND BULKING mode) to as clean choices as possible. This means you'll have to be eating MORE food (because each food item will be less calorically dense BUT it'll be higher quality food to get you where you want to be) to get to your daily calorie intake per day.
For the specific calories you should be eating a day, I'm going to let a guy or someone with major bodybuilding/competition/PLing experience jump in on that one with a specific NUMBER... if I had to guess, I'd tell you ... hmm... 3500 to start and work up to 4000 calories a day. But
please please please get a second opinion on that, okay?
What I DO know, though, is that the macronutrient ratio of what you eat daily is going to be integral to your success. This means (to track this) you should optimally record your food for about 2-3 weeks using Fitday.com to really make sure you know what's going in your body. After that, you may think you eat the same things regularly and free to stop logging for awhile. If you were prepping for a contest, however, it'd be more strict and you'd definitely need to track foods at all cost.
Anyway, the ideal macro ratio for bulking starts with... 50%p/25%c/25%f... but again, this is all trial and error.
Daily, you don't need to feed your body more than 50% of its calories through protein sources. Ideally, you want to consume your body weight in grams of protein per day, see how many calories that adds up to, and then the OTHER half of your cals comes from carbs and good fats.
Some folks feel too low energy on 50/25/25, especially when lifting heavy to gain mass/strength. Another popular option is 40/30/30... and from there you may also see some men and women jump to 40p/40c/20f if they are types to feel very low energy most of the time and desperately needing the carbs for a boost.
Sorry I'm a bit all over the place here. I hope some of this helps you out.
Let's see what others have to say.