If a team is in the huddle, a 12th player can come in but the player who is being replaced must leave within three seconds. If the huddle breaks within those three seconds, no issue. Breaking the huddle with 12 is an NCAA rule, not a high school rule.
If the official believes a team is using this to gain an advantage, an illegal substitution call can be called.
Now to your question. A no huddle team cannot have 12 line up in formation. In fact, I'm not going to wait for all 12 to get set. If they have 12 out there looing for their spot with no one making an attempt to run off the field, I'm flagging it then.
The defense will be flagged for 12 in formation, too, but they have until the snap is imminent.