Paul also talk of forgiveness of others mentioning it multiple times in his letters. In which of his letters did he discuss that subject?

He talked about it specifically in three of his letters.  In 2 Corinthians it was in regard to a specific person.  Also in Ephesians and Colossians as general advice on Christian conduct.

Source:

2 Corinthians 2:6-10, Ephesians 4:32, Colossians 3:13

One of the people who showed forgiveness in action was Stephen. His last recorded words showed his obedience.  What were they?

He asked God to forgive his executioners.

Source:

Acts 7:60

Luke also records Jesus teaching on forgiveness of others. How many times per day were they to forgive people who sinned against them?

They were to do it seven times per day. This is another way of saying as often as it takes.

Source:

Luke 17:3-4

Peter asked Jesus about forgiving his brother if he sinned against him.  Even if he did it seven times.  What number did Jesus answer Peter with?

Jesus answered Peter with seventy times seven.  Literally this is 490, but this is another way of saying without limits.

Source:

Matthew 18:21-22

Jesus had a different view of forgiveness of sins.  He commanded it in Matthew 6. What does God also promise for us, if we forgive others sinning against us?

He promised that God would forgive our sins if we would forgive others sins against us.

Source:

Matthew 6:14-15 also many other places