You can publish as many apps as you want from the same account, but if each is a pro app, then you will need a separate pro subscription for each app.
25k rows is a technical limitation. More specificly if you are using Google sheets…because of data bottlenecking issues. Beyond that and you may or may not run into speed issues, depending on how you design your app. It’s not a hard limit at the moment. You can go over 25k rows, but paying more money for the same app doesn’t remove the technical issues. It’s more or less a guideline that if you exceed 25k, you may begin so experience some issues…but not necessarily.
Each app can have it’s own domain. Your account controls all of your apps, but each app is billed separately and independent of any other app.
As a developer, am I able to demo different apps to clients with the pro features under my own domain without having to commit to a pro account until I don’t get an actual sale ?
i.e can I subscribe to one PRO app account and do all my dev/demo using that account ?
Yes you could have one PRO app account. What you could do is cancel and upgrade accordingly.
i.e. you have 5 free apps and you activate the PRO for only one at any given time. When you’re done with one demo you could cancel the subscription and upgrade another.
You get an account credit whenever you cancel a subscription.