Can a sub-account be created automatically when a client buys my plan?

Yes. When someone buys one of your SaaS plans through a payment form, their sub-account is created automatically.

1. Build a form with the Payment element.
2. Select the SaaS plan the customer checks out with. To offer several plans on one form, add them and let the buyer pick one with multi select, or build one form per plan. Both work, and you do not need a separate link for every customer.
3. Put the button at the bottom and place the form on your website.

The plan must be published before it can be used on a payment form, and publishing needs your Stripe account connected (Settings, then Integrations). You can still create sub-accounts by hand at any time from Settings, then Sub-accounts.