After someone believes in Jesus, how many good things must they do to get into heaven?
They only need to be baptized, and this is not a work they do themselves. So strictly speaking, zero.
John baptised in water, but Jesus baptized in what?
Jesus baptized through the apostles, who baptized in water, and through that baptism, they received the Holy Spirit who justified them, making them into adopted sons of God, who (like Jesus) the Father says of them: "behold my sons and daughters with whom I am well pleased".
The criminal on the cross next to Jesus was not baptized in physical water, but yet went to paradise with Jesus.
You don't know that he wasn't baptized. However though God has bound himself to the sacraments as a means of grace, He is not bound by
the sacraments and can work outside them when He chooses to.
When you say physical water is a requirement of salvation you are saying that Jesus's sacrifice wasn't enough.
No, rather, the baptism is an entering INTO Jesus' death and resurrection, and if it were not because Jesus' resurrection made it so, baptism would do nothing.
1 Peter 3:21
21 and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a clear conscience toward God. It saves you by the resurrection of Jesus Christ,
22 who has gone into heaven and is at God’s right hand—with angels, authorities and powers in submission to him.
Jesus died 2000 years ago. Do you believe that you were born saved? Why do you insist that a person must believe
? By that same measure of which you speak of baptism, a person then "adds" to Jesus' work on the cross by choosing to believe, does he not? One second, man, not saved. Next moment, man chooses to believe, man is "saved" according to some. What is the difference? A choice by man to believe, which has been "added" to the sacrifice of Christ.