The word apostle had a broad usage, but applied in a specific, tehnical sense to the 12 (later 13). Think of "president." I could be the president of the local dog lover's committee, but when sombody says "The President" I know they are not refferring to me.

Acts 1 records the appointment of Judas' successor:

Quote
21 So one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us,
22 beginning from the baptism of John until the day when he was taken up from us—one of these men must become with us a witness to his resurrection.”
23 And they put forward two, Joseph called Barsabbas, who was also called Justus, and Matthias.
24 And they prayed and said, “You, Lord, who know the hearts of all, show which one of these two you have chosen
25 to take the place in this ministry and apostleship from which Judas turned aside to go to his own place.”
26 And they cast lots for them, and the lot fell on Matthias, and he was numbered with the eleven apostles.

The technical usage of "apostle" is quite clear here- backed up by the fact that Justus actually had a higher chance of being "sent out" (Matthias would have remained in Jerusalem at that time teaching the church) and yet was not called an apostle.

(Latin phrase goes here.)