Just to expand on this, commonName is not at all required in certificates and is basically deprecated/legacy
Letsencrypt does not require you to set it, just subject alternate names, which can be up to 255 characters, but some providers require it for no reason
To further expand, commonName is only deprecated for SSL/TLS server certificates. It is, for example, mandatory for CA certificates and code signing certificates.
If a subjectAltName extension of type dNSName is present, that MUST
be used as the identity. Otherwise, the (most specific) Common Name
field in the Subject field of the certificate MUST be used. Although
the use of the Common Name is existing practice, it is deprecated and
Certification Authorities are encouraged to use the dNSName instead.
Therefore, if and only if the presented identifiers do not include a
DNS-ID, SRV-ID, URI-ID, or any application-specific identifier types
supported by the client, then the client MAY as a last resort check
for a string whose form matches that of a fully qualified DNS domain
name in a Common Name field of the subject field (i.e., a CN-ID). If
the client chooses to compare a reference identifier of type CN-ID
against that string, it MUST follow the comparison rules for the DNS
domain name portion of an identifier of type DNS-ID, SRV-ID, or
URI-ID, as described under Section 6.4.1, Section 6.4.2, and
Section 6.4.3.
9.2.2 Subject Distinguished Name Fields
a. Subject Common Name Field
Certificate Field: subject:commonName (OID 2.5.4.3)
Required/Optional: Deprecated (Discouraged, but not prohibited)
Contents: If present, this field MUST contain a single IP address
or Fully-Qualified Domain Name that is one of the values contained
in the Certificate’s subjectAltName extension (see Section 9.2.1).
Letsencrypt does not require you to set it, just subject alternate names, which can be up to 255 characters, but some providers require it for no reason