Short answer
HubSpot's KB: adaptive tests can run up to five page variations, and over time the best performer is shown more often. Every variation must include a form. All variations share the live URL with separate preview URLs. Publishing one variation makes all of them live, and after that you can edit them but not add, remove or rename variations. It needs Content Hub Professional or Enterprise.
1. Start from a page
Tests start from a published page. The landing pages list in my demo portal:

2. Design the test
- Each variation needs the form, since conversions are what the test measures.
- Change one thing per variation, such as the headline.
3. Related
Email A/B tests and landing page speed.