initialActiveIndex

<Carousel initialActiveIndex={2}>
  <Item>1</Item>
  <Item>2</Item>
  <Item>3</Item>
  <Item>4</Item>
  <Item>5</Item>
  <Item>6</Item>
</Carousel>

<Carousel initialActiveIndex={2}>
  <Item>1</Item>
  <Item>2</Item>
  <Item>3</Item>
  <Item>4</Item>
  <Item>5</Item>
  <Item>6</Item>
</Carousel>

If you are using the old prop initialFirstItem, then note that its deprecated and you should use initialActiveIndex.