Most textbooks would say the 8-12 rep range is best for muscle hypertrophy although there will be individuals and muscle groups which respond better to higher and lower repetitions. If building muscle is your goal then I think the following two guidelines are your best starting point for making progress:
- Focus on getting progressively stronger in a variety of rep ranges
- Focus on eating lots of nutritious calories to fuel muscle growth
The latter point is important, particularly for hard-gainers. Eating will become a full-time job. It's tiresome and tricky to sustain but unavoidable if you want to add significant size to your physique.