Alma 40:1 - 40:5
Alma 40: 1 Now my son, here is somewhat more I would say unto thee; for I perceive that thy mind is worried concerning the resurrection of the dead.
Alma perceives that Corianton is worried about a particular doctrine, namely, the resurrection of the dead. He wants to clear up any confusion Corianton has about it.
I think the Lord is very symphatic to people who genuinely have questions about any gospel doctrines. They can trip people up even if it is a concept that we might think that is not worth worrying over. But in general, the Lord wants everyone to be clear on any doctrine they might be fussy about. There may be speculation about a concept that has not be revealed only thought about which the Lord might not say anything about, but generally if a doctrine is well known and discussed in the scriptures, the Lord will try to clear up any confusion.
Alma 40: 2 Behold, I say unto you, that there is no resurrection—or, I would say, in other words, that this mortal does not put on immortality, this corruption does not put on incorruption—until after the coming of Christ.
Point one, the resurrection does not take place until after the resurrection of Jesus Christ.
Alma 40: 3 Behold, he bringeth to pass the resurrection of the dead. But behold, my son, the resurrection is not yet. Now, I unfold unto you a mystery; nevertheless, there are many mysteries which are kept, that no one knoweth them save God himself. But I show unto you one thing which I have inquired diligently of God that I might know—that is concerning the resurrection.
Christ will be the first one resurrected. The resurrection in general will take place after that, but Alma says it is not immediate for a person as soon as they die. Alma has been wanting to know himself just when the resurrection in general does take place. It has been a question that has troubled him and he has prayed much to receive more information as to when the resurrection for everyone does take place.
Alma 40: 4 Behold, there is a time appointed that all shall come forth from the dead. Now when this time cometh no one knows; but God knoweth the time which is appointed.
Alma knows that the resurrection for everyone will take place when it is the right time for them. It will very for each individual, but God knows.
Alma 40: 5 Now, whether there shall be one time, or a second time, or a third time, that men shall come forth from the dead, it mattereth not; for God knoweth all these things; and it sufficeth me to know that this is the case—that there is a time appointed that all shall rise from the dead.
As to whether there is a resurrection in groups at a particular time he does not know. And it really does not matter, God has it all planned out and that is good enough for Alma. All that Alma feels one needs to know is that all will be resurrection at one time or another.