Paul seems to have been the only 1st century apostle to ever have gone out of his way to proclaim his apostleship; yet he was also obviously endorsed as an apostle by the others in
Acts 15 and other passages.
Apostles do not ordain themselves. They are sent out under the authority of whoever sends them.
Revelation 2:2, tells us that there are those who falsely claim to be apostles and that the church has means for trying and discovering the veracity of the claims.