You Catholics pray to Saints but the Bible says there is only one mediator, Jesus. How can you justify this?

Jesus of course mediates a relationship with the Father in a way no one else can. No one comes to the Father except through Him. However, in terms of our relationship with him, Jesus has established things and people which help mediate our relationship with him: apostles, evangelists and teachers have roles of service to build up the body of Christ. Faith comes by hearing, hearing by the word of God. Therefore, our relationship with Jesus is mediated by both Scripture and those whom the Lord sends to evangelize us. 
 
You seem to understand "one mediator," in a completely univocal and absolute sense. If so, then you should never ask anyone to pray for you. Neither should you listen to a sermon or even read scripture. For these are things and people which mediate Christ to you in some sense. 
 
Catholics do not hold that the prayers of Saints substitute for Christ's mediation, but rather are subordinate to it, and facilitated by him. For, as Head of the Body the Church, he creates a communion of all the members, allowing, and expecting that all the members of the body assist and support one another. This does not substitute for Christ's mediation, but rather, presupposes it.