I already have an iPhone XS and if I buy the newest iPad Pro with the cellular option can it connect to my existing account with the same cell phone number or do I need a completely new number and account for the iPad?
New number but that's mainly because it has to be assigned one by carrier. You can't actually use that number for calling or texting.
If you set up Wi-Fi calling and iMessage with SMS forwarding on the iPhone, others can contact you on your iPad using the iPhone's number. I've found the feature very useful for the times I forget my phone at home. I can even get 2FA SMS from my bank.