not true, it varies based on the individual. a major part of working out is testing different rep ranges and different routines to figure how which is optimal for growth for your body. 6-8 reps is a solid rep range for gaining strength, but if you go in the gym and do that week after week your body will adapt and you will find that your gains steadily decrease.
beginners especially should start with the 12-15 rep range...this is bc they haven't lifted weights intensely at any point before in their lives, so they need to start out small by going lighter, performing more reps to strengthen and train their stabilizer muscles to even handle heavier weights, and of course build initial muscle in the first place. 12 reps will still help you build muscle. especially if you're a beginner --- hence, beginner's gains. over time though, you will have to change it up to keep making gains