How to show two columns in a CollectionView using swift4 in all devices

Make sure your protocol confirms to your ViewController,

class ViewController: UIViewController,UICollectionViewDelegate,UICollectionViewDataSource,UICollectionViewDelegateFlowLayout

And set flow layout to your collectionview object

In viewDidLoad

let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .vertical //.horizontal
layout.minimumLineSpacing = 5
layout.minimumInteritemSpacing = 5
collectionView.setCollectionViewLayout(layout, animated: true)

And implement following methods

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsets(top: 1.0, left: 1.0, bottom: 1.0, right: 1.0)//here your custom value for spacing
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let lay = collectionViewLayout as! UICollectionViewFlowLayout
    let widthPerItem = collectionView.frame.width / 2 - lay.minimumInteritemSpacing
    
    return CGSize(width:widthPerItem, height:100)
}

You must be missing UICollectionViewDelegateFlowLayout

Try this and see:

// Source code
import UIKit

class CollectionVC: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

    @IBOutlet var collection: UICollectionView!
    override func viewDidLoad() {
        super.viewDidLoad()
        collection.delegate = self
        collection.dataSource = self
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        //collection.reloadData()
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let flowayout = collectionViewLayout as? UICollectionViewFlowLayout
        let space: CGFloat = (flowayout?.minimumInteritemSpacing ?? 0.0) + (flowayout?.sectionInset.left ?? 0.0) + (flowayout?.sectionInset.right ?? 0.0)
        let size:CGFloat = (collection.frame.size.width - space) / 2.0
        return CGSize(width: size, height: size)
    }


    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 50
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "testcell", for: indexPath)
        cell.backgroundColor = UIColor.red
        return cell
    }

}

// Interface Design

Result: