If you buy Vpn can you connect multiple devices with it?

For example my cell phone and my laptop or it only works on 1 device each

Depends on the package. Some offer protection on multiple devices and some do not. I think you can protect 5 devices with NordVPN.

Depends on the provider and the package.

It should always indicate how many devices the service is valid for.

Depends on the VPN provider and the package booked. There are some providers (such as VyprVPN or Mullvad) who support a maximum of five simultaneous devices, but also providers who support more or fewer devices per subscription.