To send message from whatsapp you need to have whatsapp installed on your device if you have not installed then download whatsapp app from this link : https://play.google.com/store/apps/details?id=org.telegram.messenger
Now open the app and touch that three dots available upper right-hand side of the app you will see the options like new group, new broadcast, linked devices, started message, payments, settings.
Select new broadcast. Add number of people up to 256 contacts.
Then click on right action button now it's done.
You can send anything to them by once but you receive messages simultaneously.You can comment below if you have any queries..