Was Peter asking about salvation when he said, 'What shall we have therefore?'

No. Peter was not asking about salvation, because he already had it and knew it. He was asking what reward the disciples would receive for forsaking all and following Christ—what they would get besides eternal life. This is a valid question, and Paul deals with it in 1 Corinthians chapter 3.

Les explains that Peter’s question in Matthew 19:27 comes after Peter says, “Behold, we have forsaken all, and followed thee. What shall we have therefore?” Les qualifies that Peter was not talking about his salvation; he already had it and knew it. Instead, Peter was asking what they would get as a reward—what they would receive besides eternal life. Les calls this a valid question and notes that Paul certainly deals with it in 1 Corinthians chapter 3. The passage occurs in Jesus’ earthly ministry while the twelve, including Judas, are still with Him; the larger context shows the disciples were focused on the Kingdom of God, which Jesus had spoken of for forty days after His resurrection.

Matthew 19:27

Then answered Peter and said unto him, Behold, we have forsaken all, and followed thee; what shall we have therefore?

What did Peter mean when he asked Jesus, 'What shall we have therefore?'

Peter was not questioning his salvation, which he already possessed and knew. He asked what reward the twelve would receive for forsaking all and following Jesus—what they would get besides eternal life. It was a valid question about future reward, later addressed by Paul in 1 Corinthians 3.