is there any way to make the shocks stiffer in my rustler?

Put the spring clips on that come in the kit, change the springs, change the oil to thicker oil, change the pistons inside the shock to ones with fewer/smaller holes, change the mounting of the shocks to a more upright location.

how many clips can you put on one shock?

as many as you can fit on the body. they come in different thicknesses so the number varies, but what you really need to do is put stiffer springs on there, then maybe some heavier oil

Thicker oil doesnt make it stiffer/softer.It makes the response of the shock faster or slower.
Stiffer springs or preload spacers makes it stifffer.

no kidding. Never claimed the oil makes it softer/stiffer, but a thicker oil will definitely compliment a stiffer spring

if you dont know how to change the oil just buy different stiffer shocks or put the c clip spacer things in that way you can modify it to your liking also