periodization

if the next set i can do less reps with the same weight, do i still need to do periodization?

If you're a beginner, you don't need to do periodization at all.  When you need to do it, you'll know that you need to do it:  it's when progress stalls despite every change you try.  A good periodization scheme changes "everything" about your training sessions on a planned schedule, trying like hell to avoid adaptation to fixed routines.

Advanced lifters need this, and it's much more involved than just dropping the number of reps across sets.  The body will adapt to any single gimmick like that over time, and will adapt quicker the more progress you make.  If you get to the point where you stall every few weeks, periodization can be essential to speed progress.

BTW, that doesn't mean you shouldn't do periodization near the start of your journey -- "don't need" isn't saying "shouldn't".  The closer you are to the start of this journey, the easier it is to shock muscles into growing, so the less need for fancier schemes.

periodization is breaking up your workouts over the course of a long time, normally a year. after about 6 months of beginner lifting you should sit down and write down your goals and take a scientific approach to things. Most bodybuilders will do there bulking in the winter and leaning out in the spring or a contest that is coming up. but you should have a plan of attack.