Were there always 12 apostles

Yes. Apostle is the title Jesus gave to His closest circle of friends, the Twelve (Luke 6:13). After the first Easter, the early church expanded the term to refer to a wider circle of authoritative preachers and witnesses of the resurrected Lord. The criteria employed for replacing Judas among the Twelve included being an eye witness not only of the resurrected Jesus but also of the ministry of Jesus from the days of His baptism by John. Paul and the early church developed a slightly broader application of the term apostle that did not demand an eyewitness knowledge of Jesus' ministry. Jesus gathered a special circle of Twelve, clearly a symbolic representation of the Twelve Tribes of Israel. He was reestablishing Jewish social identity based on discipleship to Jesus. Disciple as a reference to the Twelve became an exact equivalent to apostle in those contexts where the latter Word was also restricted to the Twelve.