How to monitor more than 20 regions?

There is no way to monitor more than 20 regions using Apples API.

You have to update the actively monitored regions to the nearest 20 regions.

Whenever you enter/leave a region:

• Check entered location
• Stop monitoring all regions
• Start monitoring the nearest 19 regions (distance to entered location) plus the entered one.

If the result is not satisfying, you might also want to monitor significant location changes to have the chance to update the monitored regions every ~500 meters while not draining too much battery.

set currentLocation from your didUpdateLocations

var currentLocation : CLLocation?{
    didSet{
        evaluateClosestRegions()
    }
}

var allRegions : [CLRegion] = [] // Fill all your regions

Now calculate and find the closest regions to your current location and only track those.

func evaluateClosestRegions() {

    var allDistance : [Double] = []

    //Calulate distance of each region's center to currentLocation
    for region in allRegions{
        let circularRegion = region as! CLCircularRegion
        let distance = currentLocation!.distance(from: CLLocation(latitude: circularRegion.center.latitude, longitude: circularRegion.center.longitude))
        allDistance.append(distance)
    }
    // a Array of Tuples
    let distanceOfEachRegionToCurrentLocation = zip(allRegions, allDistance)

    //sort and get 20 closest
    let twentyNearbyRegions = distanceOfEachRegionToCurrentLocation
        .sorted{ tuple1, tuple2 in return tuple1.1 < tuple2.1 }
        .prefix(20)

    // Remove all regions you were tracking before
    for region in locationManager.monitoredRegions{
        locationManager.stopMonitoring(for: region)
    }

    twentyNearbyRegions.forEach{
        locationManager.startMonitoring(for: $0.0)
    }

}

To avoid having the didSet called too many times, I suggest you set the distanceFilter appropriately (not too big so you would catch the region's callbacks too late and not too small so that you won't have redundant code running). Or as this answer suggests, just use startMonitoringSignificantLocationChanges to update your currentLocation