How many lanes in the 100 meter dash?

It depends on the pool. Most competitive swim meets take place in 8-lane or 10-lane pools. Meets taking place in 10-lane pools typically fill the entire pool for the preliminary heats. In other Words, all 10 lanes are taken up by swimmers. However, in the semi-finals and finals, only 8 lanes are used. This keeps the two outside lanes empty.

However, meets can take place in pools of any size. There can be 6-lane pools or 4-lane pools. It is not always the same.