1#header-token
みたいなのは「少なくとも1個以上のheader-tokenがカンマ区切りのリストとして現れる」と読める。
出典:
A #rule extension to the ABNF rules of [RFC5234] is used to improve
readability in the definitions of some header field values.A construct "#" is defined, similar to "*", for defining
RFC 7230 - Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
comma-delimited lists of elements. The full form is "# element"
indicating at leastand at most elements, each separated by a
single comma (",") and optional whitespace (OWS).