When I activate the ability of jhin legend and already have 12 Runes, does Channel mean ready as well?

No, "channel" does not mean "ready." According to RiftJudge FAQ #2680, channeling specifically refers to taking one or more runes from the top of your Rune Deck and placing them onto the board (Rule 430.1). It is not equivalent to readying runes already on the board.

Regarding the scenario with the Jhin Legend card (Virtuoso):

1. Does Channeling Ready Runes?: No. As established by Rule 430 and FAQ #2680, channeling is the process of adding new runes from your deck to the board. It does not ready any runes that are already present on your board.
2. Maximum Rune Constraint: According to RiftJudge FAQ #5452, if you are at 12 runes (the maximum), you cannot channel additional runes. If you cannot channel, the effect simply fails to channel those runes. In this specific case, since you are at the maximum of 12 runes, the instruction to "channel 4 runes" from the ability of Virtuoso would not result in any runes being placed on the board, nor would it cause any of your existing runes to ready.

In summary, the ability requires you to channel, but channeling does not ready existing runes, and if you are already at the maximum rune count, the channel portion of the ability cannot be executed.

Note: I could not fully verify this ruling against the retrieved rules.