For iOS its showing that we can't use more than 10 waypoints to draw a route (including source and destination waypoints).
for android there is no limit for the waypoints.
I used the following code to draw a route :
func getDirections(origin: String!, destination: String!, waypoints: Array<String>!, travelMode: AnyObject!, completionHandler: ((_ status: String, _ success: Bool) -> Void)?) {
        if let originLocation = origin {
            if let destinationLocation = destination {
                var directionsURLString = baseURLDirections + "origin=" + originLocation + "&destination=" + destinationLocation
                if let routeWaypoints = waypoints {
                    directionsURLString += "&waypoints=optimize:true"
                for waypoint in routeWaypoints {
                    directionsURLString += "|" + waypoint
                }
            }
            print(directionsURLString)
            directionsURLString = directionsURLString.addingPercentEscapes(using: String.Encoding.utf8)!
            let directionsURL = NSURL(string: directionsURLString)
            DispatchQueue.main.async( execute: { () -> Void in
                let directionsData = NSData(contentsOf: directionsURL! as URL)
                do{
                    let dictionary: Dictionary<String, AnyObject> = try JSONSerialization.jsonObject(with: directionsData! as Data, options: JSONSerialization.ReadingOptions.mutableContainers) as! Dictionary<String, AnyObject>
                    let status = dictionary["status"] as! String
                    if status == "OK" {
                        self.selectedRoute = (dictionary["routes"] as! Array<Dictionary<String, AnyObject>>)[0]
                        self.overviewPolyline = self.selectedRoute["overview_polyline"] as! Dictionary<String, AnyObject>
                        let legs = self.selectedRoute["legs"] as! Array<Dictionary<String, AnyObject>>
                        let startLocationDictionary = legs[0]["start_location"] as! Dictionary<String, AnyObject>
                        self.originCoordinate = CLLocationCoordinate2DMake(startLocationDictionary["lat"] as! Double, startLocationDictionary["lng"] as! Double)
                        let endLocationDictionary = legs[legs.count - 1]["end_location"] as! Dictionary<String, AnyObject>
                        self.destinationCoordinate = CLLocationCoordinate2DMake(endLocationDictionary["lat"] as! Double, endLocationDictionary["lng"] as! Double)
                        let originAddress = legs[0]["start_address"] as! String
                        let destinationAddress = legs[legs.count - 1]["end_address"] as! String
                        let originMarker = GMSMarker(position: self.originCoordinate)
                        originMarker.map = self.mapView
                        originMarker.icon = UIImage(named: "mapIcon")
                        originMarker.title = originAddress
                        let destinationMarker = GMSMarker(position: self.destinationCoordinate)
                        destinationMarker.map = self.mapView
                        destinationMarker.icon = UIImage(named: "mapIcon")
                        destinationMarker.title = destinationAddress
                        if waypoints != nil && waypoints.count > 0 {
                            for waypoint in waypoints {
                                let lat: Double = (waypoint.components(separatedBy: ",")[0] as NSString).doubleValue
                                let lng: Double = (waypoint.components(separatedBy: ",")[1] as NSString).doubleValue
                                let marker = GMSMarker(position: CLLocationCoordinate2DMake(lat, lng))
                                marker.map = self.mapView
                                marker.icon = UIImage(named: "mapIcon")
                            }
                        }
                        let route = self.overviewPolyline["points"] as! String
                        let path: GMSPath = GMSPath(fromEncodedPath: route)!
                        let routePolyline = GMSPolyline(path: path)
                        routePolyline.map = self.mapView
                        routePolyline.strokeColor = UIColor(red: 44, green: 134, blue: 200)
                        routePolyline.strokeWidth = 3.0
                    }
                    else {
                        print("status")
                        //completionHandler(status: status, success: false)
                    }
                }
                catch {
                    print("catch")
                    // completionHandler(status: "", success: false)
                }
            })
        }
        else {
            print("Destination is nil.")
            //completionHandler(status: "Destination is nil.", success: false)
        }
    }
    else {
        print("Origin is nil")
        //completionHandler(status: "Origin is nil", success: false)
    }
}