I got no extra ones with ds, and he was almost a pound heavier than dd, and 6cm longer. I don't think there was any room left for new ones tbh . I worried about it too but it was fine, and they weren't red and angry looking like first time round, they stayed pretty faded
My dc like to spot their own lines on my tummy. DD's went up and down, ds1's are sort of diagonal either side (he was biggest), ds2 was breach and only caused one right at the top of my tummy where his head was.
I don't really care about them as my bikini wearing days are well and truely over so the only person to really see them is dh and I blame him for not passing on the genes for little babies!