You can only create a small number of gmail accounts, since ever account needs to be linked to a valid phone number. Google actively work to prevent using their platform in this way.

You can also pay $2.40 for a domain name for a year and have your own disposable email service.

You actually don't need have to have a phone number to use Gmail. You can skip that step.

I don't think so. I have numerous old Google accounts with their passwords in my password manager. Not for gmail, but for Google Groups lists, Google+ and various other services that no longer exist. Whenever I log in (in an account container of course) I cannot continue without adding a phone number. Have not found a way to skip the step (well, have not tried for some months now, trying to avoid Google increasingly). None of the accounts has stored any data. I would understand that Google would block people from misusing them as free cloud storage.

Where could I get disposable phone numbers from at a reasonable cost? So that I can receive just the first SMS. These are not valuable accounts, I don't need password reset years later.

You can use protonmail.