Volt customers are entitled to 3 mesh pods, free of charge. If you're not a volt customer then you pay £5 per month for each one. Unless you're on the ultimate oomph package.
VM will only send one out at a time, easiest way to order is to call them, you can order them via the intelligent WiFi app too, but that can be a little hit and miss.
Full volt benefits are described here:
https://www.virginmediao2.co.uk/