I actually had a whole paragraph about junctions being a limit and then deleted it since i didn’t feel like it added to my point. I also was going to add a point about how much space the lanes take up and that even if more lanes added capacity, it didn’t necessarily mean they were the right option.
It does increase the capacity of roads. Two lanes holds twice as many cars as one lane. Four lanes hold twice as many cars as two lanes.
You’re probably thinking of induced demand, but that’s related to traffic congestion and not capacity. More lanes ultimately means more cars are getting places, but any individual car will see that congestion is just as bad as it used to be.