You can add multiple, but practically speaking you're better off with used 3090s which you get 2 for the price of one 4090.
I have 3090 Ti and I can run Q4 quant 33b models at 30t/s with 8k context. A 4090 would allow me to do the same but with ~45t/s, both inference speeds are more than fast enough for people so 3090 is the usual choice. In my tests on runpod, H100 with 80GB memory is around the same speed as 3090, so slower than a 4090.
I have 3090 Ti and I can run Q4 quant 33b models at 30t/s with 8k context. A 4090 would allow me to do the same but with ~45t/s, both inference speeds are more than fast enough for people so 3090 is the usual choice. In my tests on runpod, H100 with 80GB memory is around the same speed as 3090, so slower than a 4090.