Question

insert two 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 can be used to set off additional information. Here, "some five thousand paddlers in total" is extra - information about the teams, so dashes are inserted around it.

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.