Thanks @rashid779939, yes I saw that. The conclusion was that “No, there is no limit to the number of applications that can be created in an Auth0 account.” (and then a broken link).
So, it seems that the situation with applications per tenant has changed in the meantime.
In fact, now I try this via the Auth0 Dashboard (instead of terraforming) I see a message like this:
This tenant reached the limit of 10 available applications and SSO Integrations per the [Auth0 Entity Limit policy](https://auth0.com/docs/policies/entity-limits). You can increase the number of available applications and SSO Integrations by upgrading your [ Auth0 subscription](https://manage.auth0.com/dashboard/eu/<redacted>/tenant/billing/subscription) to a paid plan. Please [ contact us](mailto:firstname.lastname@example.org) with any questions.
Like I say though, the plans page gives no details of by how much the limits are changed depending on the plan.