Question

insert dashes where needed. at the international dragon boat carnival in hong kong, hundreds of teams from around the world some five thousand paddlers in total race to the sound of beating drums.

Explanation:

Dashes are used to set off an additional description. Here, "some five thousand paddlers in total" is additional information about the "hundreds of teams".

Answer:

At the International Dragon Boat Carnival in Hong Kong, hundreds of teams from around the world—some five thousand paddlers in total—race to the sound of beating drums.