Ephesians 6:18King James Version (KJV)
18 Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and
supplication for all saints;
1 Timothy 2:1-4King James Version (KJV)
2 I exhort therefore, that, first of all,
supplications,
prayers,
intercessions, and giving of thanks,
be made for all men;
2 For kings, and for all that are in authority; that we may lead a quiet and peaceable life in all godliness and honesty.
3 For this is good and acceptable in the sight of God our Saviour;
4 Who will have all men to be saved, and to come unto the knowledge of the truth.
Clearly the bible tells us in many places that we are to pray and intercede for one another. The Lord does not say his answer or ability to hear those prayers is based on numbers, he simply says we should pray for others. It is that simple really.
With that being said, Jesus himself showed us this example.
Luke 22:32King James Version (KJV)
32 But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren.
Jesus prayed for Peter. He is our example, he is our Lord so we have a pattern.
Also, I do not ask just anyone to pray for me personally.
James 5:16King James Version (KJV)
16 Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.
The bible says the prayers of the righteous avail much. If I know a righteous person I will ask them for prayer. But if I know a brother is carnal and immature I generally do not ask them to pray for me. Who knows what they are praying in that case, IMO. Also, I do understand that most people unfortunately say they will pray but never do. This is like people, especially unsaved who say things like "praying for you" or I will "pray for you".
Praying for others produces humility in ourselves. We are sacrificing our own time for another. We are interceding for another putting our own needs aside. It also helps with selfishness and pride as well