From their docs[0] this doesn't seem to apply if using ACME, but they don't exactly make that clear...
> By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards. Each certificate you create will be stored in your ZeroSSL account.
> By using ZeroSSL's ACME feature, you will be able to generate an unlimited amount of 90-day SSL certificates at no charge, also supporting multi-domain certificates and wildcards. Each certificate you create will be stored in your ZeroSSL account.
[0]: https://zerossl.com/documentation/acme/