The reason why I thought that most of the herd died outside of the grandparents and Littlefoot's mother, upon Littlefoot's birth, is what the narrator says.
"All that remained of his herd was his mother, his grandmother, and his grandfather."
The "all that remained of his herd" is what makes me believe the others are likely dead. Littlefoot's herd may never have been that large to begin with, and the landscape was harsh, not to mention that, if Sharptooth himself is any evidence, sharpteeth in this area were downright nasty and vicious and tough to beat.
The phrase "all that remained of his herd" doesn't make it sound like, to me, that members of his herd just left; it sounded like the narrator meant it in a literal way; that only a few longnecks in the herd were still alive. If he had said something like "all that remained of his immediate family" or something along those lines, then I would have been more inclined to think that some herd members survived.
While yes Bron may technically prove the narrator wrong in this since he is alive, I don't know if Bron was ever a member of the herd. A possibility is that Littlefoot's mother left the herd to join Bron and they were going to start a new herd, but then Bron left to find a new home so Littlefoot's mother had to rejoin her own herd. Maybe not the most likely of possibilities I admit.
Even so, Bron's survival doesn't mean that there were many more survivors, if any. The landscape wasn't very hospitable and food was very scarce. The grandparents' survival may have been just luck. But then again, we do see that they are still fully capable of fighting sharpteeth in the sequels, so it stands to reason that, despite being old, they were still tough, so maybe it isn't so surprising they made it to the Great Valley. They may have been much more experienced with fighting sharpteeth and dealing with harsh conditions that gave them an edge over most of the herd.
But in the end, it's just speculation. And it's doubtful we'll ever find out what actually happened.
