Yeah, only 21, but maybe if they rotate... If not you're stuck with backnumbers. If they're honest, it can be a decent alternative since it's at least permanent. If they're really honest and nice, they'll give you the entire backlog without removing anything, but I know exactly one creator who does this.
Also, if you're lured by the backnumber campaign, the coins are only handed out the following month (and it's just 100 JPY anyway). They don't want you to unsubscribe, lol.
The upside is this is the only platform I know that has fully public comments.