Updated: Nov 7, 2020
There are 2 "types" of intensity:
Intensity of load - how much you're lifting (generally based on a % of 1RM).
Intensity of effort - how close to failure you're training.
Intensity of load is not very important for muscle growth, as long as you stay within certain boundaries. One study  showed that very high-load sets (2-4 reps per set) resulted in less muscle growth when compared to moderate loads (8-12 reps per set), when the number of sets is equated. Then, we have a study  that found that sets of 8-12 reps and 25-35 reps were equally effective at building muscle. Finally, one meta-analysis  concluded that various rep ranges can be used if your goal is muscle growth.
OK, so we know that we should stay away from really high-load sets (sets with 4 or less reps), and that we can go quite low in load. But how low can we go, without compromising muscle growth? Well, this study  found that using only 20% of 1RM resulted in significantly less gains when compared to using 40%, 60% or 80% of 1RM for the biceps and quads. So, maybe you should stay above 30-40% of 1RM (meaning less than ~30/35 reps per set). Very high-rep sets near true failure are very uncomfortable, which increases the probability of stopping far from failure. For that reason, I advise you to stay below 25/30 reps per set.
Bottom line, as long as you don't use extremely low or extremely high loads, you're fine. This means staying in the 5-30 rep range.
Now, onto intensity of effort. This "type" of intensity is of great importance for muscle growth . You need to make sure each set you count towards your weekly volume is hard, meaning close to failure. You don’t need to go to actual failure (I’ll write another article on this), but you have to stay within a few reps from it, maybe up to 4/5. This is where the RIR (reps in reserve) scale enters, which is used to assess proximity to failure:
0 RIR = failure (you had no more reps in you)
1 RIR = 1 rep in reserve (you had 1 more rep in you)
2 RIR = 2 reps in reserve (you had 2 more reps in you)
For each set you attribute a given RIR, according to how close to failure you were. This is a subjective scale, so you have to be really honest with yourself and also know what training to failure feels like, because that's the reference.
Overall, perform each set with ~5-30 reps @ 0-4/5 RIR.
P.S. If you're a complete beginner, you should focus on practicing good technique first. That's really important because when a set gets really tough, technique tends to get sloppy and, if you don't have a solid technique, the probability of that happening is much higher. Once you've mastered the movement, you can start working closer and closer to failure.
 Schoenfeld BJ, Contreras B, Vigotsky AD, Peterson M. Differential effects of heavy versus moderate loads on measures of strength and hypertrophy in resistance-trained men. J Sports Sci Med. 2016 Dec;15(4):715-722.
 Schoenfeld BJ, Peterson MD, Ogborn D, Contreras B, Sonmez GT. Effects of low- vs. high-load resistance training on muscle strength and hypertrophy in well-trained men. J Strength Cond Res, 2015 Oct;29(10):2954-63.
 Schoenfeld BJ, Grgic J, Ogborn D, Krieger JW (2017). Strength and hypertrophy adaptations between low- vs. high-load resistance training: A systematic review and meta-analysis. J Strength Cond Res. 2017 Dec;31(12):3508-23.
 Lasevicius T, Ugrinowitsch C, Schoenfeld BJ, Roschel H, Tavares LD, De Souza EO, Laurentino G, Tricoli V. Effects of different intensities of resistance training with equated volume load on muscle strength and hypertrophy. Eur J Sport Sci. 2018 Jul;18(6):772-780.
 Morton RW, Colenso-Semple L, Phillips SM. Training for strength and hypertrophy: an evidence-based approach. Curr Opin Physiol. 2019 Oct;11:149-150.