what number of sets to reps should i do for dips if i can't do them weighted for gaining mass?
The general parameters of sets to reps for bw dips isnt going to matter as much for gaining mass as extra calories will. Thats really the key for hypertrophy. Are you saying you cant do weighted dips? If so, your body weight is obviously presenting a challenge enough, right? At any rate, you could experiment and see-maybe start off with 2 sets of your max or 3. This is also a very loose end question. After all, are you merely going to the gym to do dips? Is this your main chest movement? Are you doing this to hit your tri's on a split system after your chest? Are you doing this as a secondary compound movement to your chest training, be it a full body or split system? these are all variables that can affect the volume of sets you would do. you might get a larger response if you are a little more detailed with your question.
Go for 5x5 and see if you can complete that.