I don't fully know the rules, but I served on a 6-person jury in a civil cast in Florida. When it was time to deliberate, the judge released the one alternate juror. (We had no idea who the alternate was until he was released.) At that point, he left and so became "contaminated" by outside influences, so I'm not sure how an alternate could be called back.
I have been on a criminal jury that was split 10-2 for acquittal, the judge refused a mistrial and we kept deliberating and reached a verdict.
I promise you that any commenter here who “knows what they would have done” from reading news reports and internet slop has never been on a criminal jury.