It is not possible to use Whatsapp on more than one devices with the same number.
When you verify your number, your Whatsapp installation is linked with your number. After this, Whatsapp stays active and verified no matter which sim card or number you use. That means, even if change the sim card/number you used for verification, Whatsapp continues to work. You can use the old number in any new phone and your Whatsapp on your old phone will continue to work.
A common misconception here is that you can re-verify/re-activate Whatsapp installation on your new phone. Since Whatsapp is already working on the old phone, verifying it again on the new phone will have two Whatsapp installations working simultaneously. This is a wrong opinion. The moment you verify Whatsapp on your new device, installation on the old device no longer works. Whatsapp probably takes the IMEI number os some installation ID while linking it with your number. The moment it has a new IMEI/ID linked with your number, the old linked IDs will be deactivated.
It is also not a good idea to switch phones and activate Whatsapp again as and when you need. Whatapp on their website mentions that if you switch devices frequently, you will be blocked from re-verifying at a certain point.