It is a one-time purchase per platform. So, iOS is $5. Android is another $5.
You can install it on multiple phones and tablets if you'd like. Just use the same credentials to sign into Pushover on all devices, so they are all tied to the same Pushover account that you used to buy the iOS or Andorid license. Doing so, will create a new device in your Pushover account for each device it is installed on.
On the Hubitat side, you 'Add a Virtual Device', select the Pushover Driver, and then fill in your Pushover API key(s). After doing so, the device will query the Pushover server and return a list of 'pushover devices' which you can select from, or leave it as is to have notifications sent to all of your pushover devices at once. You can create as many Hubitat Pushover devices as you'd like (e.g. one for ALL devices, and others for each unique device.) I have a "Pushover-All" device, and others for my phone, my iPad, my wife's phone, and another for her iPad. This provides flexibility when sending notifications from various apps. It is also possible to send Pushover notifications from multiple Hubitat hubs, as well as from Node-RED. I prefer the flexibility Pushover provides over the Hubitat mobile app's push notifications, that only work with one Hubitat hub.