In his recent video about chest, Scott said that 12-14 working sets is enough to see growth.. However I've heard big muscles such as ches, legs.. etc require 20-25 working sets. Is it true? Isn't 20 sets too much for a muscle.. Does it not count as overtraining it? 

Overtraining is very hard to do. And really, it's actually more like under-eating or under-recovering more than anything. I was talking about 12-14 sets for one workout. Then you might hit chest a second time in the week to get more volume and get towards that 20 set range. Doing 20 sets isn't too much for the bigger muscles.

Alright man. I'm actually hitting chest once with 15 working sets per week

OK nice. That's definitely not too much! If you feel like you're not seeing great results with training it once a week, then maybe pick a second day to do a bit more training for more frequency and more overall volume. Some Cheat & Recover will get it growing!! https://muscularstrength.com/cheat-and-recover

Definitely! Also how long do you think a workout should be? They say anything above 1hr is overtraining

Depends how many muscle groups you are training, how long your rest periods are etc. You can get a solid workout done in an hour, put it that way. A one hour workout is a good target, but going up to 90 minutes is OK as well. Honestly I am sometimes in the gym for more like 2 hours, incuding mobility warm-ups and abs training and things like that.

So pretty much if you are working out for 1-2 hours that's normal when training multiple body parts. You probably wouldn't need to spend 2 hours on just chest for example. Again, it's just about making sure you eat enough/rest enough to compensate for the training.

Good it man! My push day is about 1hr 15 mins and so is my pull day. Leg workout is about 1hr. I thought training more than 1hr is detrimental XD but now i know its really not

Those sound like average times, so you are on track bro! Nice! Yeah theres's nothing wrong with it.. just make sure the diet and sleep are right and keep it up! 😁