If I buy Notability for my iPad pro, will it be a 'one time payment' or will I have to revise my payment every month like that in Evernote?

Evernote mentions it in its description it has Subscription options, as well as the list of in-app purchases further down in its details.

Evernote on the App Store

EVERNOTE PLUS - More space. More devices. More freedom.

• 1 GB of new uploads each month

• Unlimited number of devices

• Access your notes and notebooks offline

• Save emails to Evernote

$3.99 monthly, $34.99 annually

EVERNOTE PREMIUM - The ultimate workspace.

• 10 GB of new uploads each month

• Unlimited number of devices

• Access your notes and notebooks offline

• Save emails to Evernote

• Search inside Office docs and attachments

• Annotate PDFs

• Scan and digitize business cards

• Show notes as presentations, instantly (desktop only)

$7.99 monthly, $69.99 annually

Evernote offers paid monthly subscriptions for added features in their Plus and Premium packages. However, you can choose not to pay for those and stick with the basic free App with no added features and not be charged every month if you don't want to. The monthly payments are entirely optional.

Notability on the other hand has a one time Price, but no other subscription options, although it does offer some in app purchases for display themes. Again you are free to choose what you want to pay for. You don't need to pay for those if you do not want to. But you will need to pay the one time purchase Price to even use the App.

It says it’s 9.99 with the in app purchases looking like theme packs

Evernote doesn’t specifically mention being a subscription. However this is a user to user forum so the best place for you to direct your question is the developers of the app themselves. Only they can tell you all the costs associated with their app.

If you don’t need the theme packs, Notability is a one-time purchase. No monthly subscription.