UICollectionView没有显示正确的项目数

Try specifying the size for item at indexPath, to do that make sure you have set self.calendar.delegate = self and then add an extension to your ViewController

extension ViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: self.calendar.bounds.size.width / 7, height: your_cell_height)
    }
}

This will work assuming you haven't specified any spacing between the cells, if you have specified spacing between the cells make sure you account it as well in your calculation.

For example: if you want spacing between cells (horizontally) be 10 calculation will change to

(self.calendar.bounds.size.width / 7) - 60) why 60? ( 6 spaces between 7 cells each of size 10 so 6 * 10)

Finally, why is it showing 7 cells in XIB and not in real device/ simulator? XIB's screen width might be different from the device screen width, you can drag Xib to required width to accommodate as many cells as you want horizontally, that does not guarantee that when app runs in real device it gets the similar real estate.

它是一种流布局，因此内容会流，当它意识到没有更多空间可容纳一行中的更多单元格时，内容（单元格）会流到下一行，希望可以澄清问题

PS：切勿使用Pascal大小写（第一个字母为大写）来命名变量，请@IBOutlet weak var Calendar: UICollectionView!始终使用Camel大小写更改为@IBOutlet weak var calendar: UICollectionView!