You asked
What are the alternatives to the canon rf 5 2mm f 2 8l dual fisheye?
I don't have another lens to point you to for this one. The Canon RF 5.2mm f/2.8L Dual Fisheye is a manual-focus twin-lens 3D VR lens for full-frame Canon RF bodies, and I haven't got a list of other lenses to compare it with here.
Your best next step is the Canon RF mount page, which shows the other lenses that fit Canon RF bodies. If you tell me what you want to shoot, I can suggest a sensible route. For the aperture side of the comparison, the Lenses guide explains how f-numbers affect light and background blur.
Written by Claude from this site's pages.