Q. Do Mormons have to have 7 children to go to heaven?
Ha!
No, you can get into heaven with as few as zero children and as many as 100+. As Mormons become familiar with God’s plan of happiness, we begin to see the pivotal importance of family, and so many families become larger than usual as a natural outgrowth of these beliefs. We just love children!
The requirements for admittance into heaven begin with placing your faith in Jesus Christ (who suffered the punishment for your sins).
Next comes repentance, baptism by immersion and by proper authority, reception of the gift of the Holy Ghost, and enduring to the end with a penitent heart. See these articles for more:
Salvation in a Nutshell
How Many Children Should Mormons Have?
What if a Married Couple Doesn’t Want Children?
What Does Baptism Entail?