I accidentally in your base

I suggest rails and brackets every 30-40cm. Contiboard is not overly stiff. Ensure you use strong wall fixings; embed them into the wall material not just the surface plaster. I have successfully mounted those sort of rails with heavy loads using holes drilled into the brick of the wall with fairly long screws and correctly-sized rawlplugs. It takes a strong arm or a power driver to get the screws in.
I got my black contiboard shelves from a local large builders' merchant, who could also do board cutting. They sell it in 4' x 8' pieces (... metric equivalent, but we all know what the odd metric numbers are based on). I had made out a simple cutting plan, and got them to use this, so I got two long shelves off each long side of the large piece with some waste in the middle. One can also get black edging strip to iron onto the pieces to use parts cut from the middle of the large pieces.
My method for ensuring it was all level was to mount one rail vertically (using spririt level), then to put a bracket in and a shelf on, then arrange a rail and bracket at the other end and moving it until the shelf is level and fixing it on the wall, then adding the intermediate rails adjusted to the shelf position. Much easier than trying to measure the position of everything up front.