Automatically increment the iOS build number in Unity conforming to Apple's guidelines before every build. Be sure to drop into a folder name "Editor". Worked in autumn 2020.

IncrementBuildNumber.cs

using UnityEngine;
using UnityEditor.Build;
using UnityEditor.Build.Reporting;
using UnityEditor;

public class IncrementBuildNumber : IPreprocessBuildWithReport {
	public int callbackOrder { get { return 0; } } // Part of the IPreprocessBuildWithReport interface

	public void OnPreprocessBuild(BuildReport report) {
		if (report.summary.platform == BuildTarget.iOS) {

			// to update major or minor version, manually set it in Edit>Project Settings>Player>Other Settings>Version
			string[] versionParts = PlayerSettings.iOS.buildNumber.Split('.');
			int buildNumber = 0;

			if (versionParts.Length != 3 || !int.TryParse(versionParts[2], out buildNumber) ) {
				Debug.LogError("BuildPostprocessor failed to update version " + PlayerSettings.bundleVersion + " with build number: " + PlayerSettings.iOS.buildNumber);
				return;
			}
			// major-minor-build
			versionParts[2] = (buildNumber + 1).ToString();
			PlayerSettings.iOS.buildNumber = PlayerSettings.bundleVersion + "." + versionParts[2];
		}
	}
}