Does it take longer to cook more pork ribs?
It's not entirely clear what you mean, but if you mean the difference between cooking 1 rack or 6, the answer is that there isn't one. The oven will heat up six racks just as quickly as one, unless you stack all of them directly on top of each other or something like that. The only thing you have to adjust is the seasoning. Otherwise, just make sure they have space and cook like it was one rack.