One way of looking at this is to think of the phone number registered in WhatsApp as a login id that links a device to that account on WhatsApp. The device can be a phone (typically) but can also be a tablet or PC with a WhatsApp client app installed. I have these on an Android tablet and Windows laptop. You can have up to four of these devices active, your primary phone and three secondary devices, and you will be able to synchronise them.
There are some limitations to this. Two that I know of are:
- The primary phone has to be used no less than every fourteen days, otherwise all the synced devices are logged off, and will need to be set up again.
- Some message content, like photos, does not sync automatically, but can be downloaded to the secondary devices
I suspect most people know this, but it might be worth repeating. WhatsApp only requires an IP (internet) connexion. This can be through the mobile network you are connected to, or any other internet connexion you can find, which these days is typically WiFi. So if you want to preserve the capacity from your mobile provider, it is possible to go to flight mode, enable WiFi, and only use WhatsApp at bars, cafes, albergues and anywhere that offers WiFi.
That said, if you want to use a service like AlertCops in Spain, you will need to either keep your mobile out of flight mode, or turn flight mode off when you want to use it. That would be an extra step to think about when something is happening you want to report quickly.