The answer seems to be there is no hard limit on the number, but after awhile they disallow you to use the same phone number to authenticate accounts so you would have to use different numbers.
https://support.google.com/accounts/...e-number?hl=en
Quote:
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The exact limitations on the number of Gmail accounts you can verify using a single mobile number are not explicitly mentioned by Google. However, users have observed that there is a general restriction of about four accounts per mobile number. This limitation helps prevent abuse and misuse of the platform.
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https://mailmeteor.com/blog/how-many...nts-can-i-have