Additionally, what other sites in Europe or elsewhere have a reasonable claim to being the burial site of an apostle or early church leader, and what evidence is there for those sites? For example, there's a legend that Saint Matthew passed through Central Asia on his way to India, and founded a monastery on the shores of Lake Issyk-Kul, now in Kyrgyzstan. Is there any support for this beyond local legends? Ancient textual references or archaeological evidence?
The word "apostles", has a somewhat vague and variable definition.
Some use it to refer to the 12 disciples of Jesus. St. Paul, who was not one of the 12, called himself an 'apostle'. Some call the "70 Disciples" sent out by Jesus to spread the gospel (Luke 10:1) "apostles". Some call any famous (and particularly, evangelizing), church leaders, such as St. Patrick, "apostles".
The 12 Disciples were: Simon Peter, Andrew (Peter's brother), James, John (James' brother), Philip, Bartholomew (except that the gospel of John names "Nathanael" rather than "Bartholomew"), Thomas, Matthew, James (son of Alphaeus), Thaddaeus or Jude ("Thaddaeus" in Matthew and Mark, "Jude" in Luke and John, "Judas (son of James)" in Acts), Simon ("The Cananean" in Matthew and Mark; "The Zealot" in Luke and Acts), Judas Iscariot (died after betraying Christ and replaced (by the other disciples) by Matthias, according to Acts.
Not even the gospels agree on who the original 12 disciples were.
So, all that confusion might explain how 17 apostles are claimed to be buried in Spain, (but I don’t know how 17 out of 12 apostles could be buried in Spain unless some are thought to be buried in more than one place).
This might well be the case, as the remains of St. Mark (an Evangelist, but not one of the original 12) are claimed to be divided between Alexandria (where he is said to have been martyred) and Venice (whence some Greek monks smuggled his body – minus, according to legend, his head (which is still said to be in the Coptic church in Alexandria) after Alexandria fell to the Muslims).
Anyway, as far as I am aware, Christian tradition only identifies the burial sites of 7 of the original 12 disciples. In Christian tradition, all 12 were martyred, except John, who survived into old age.
Simon Peter is thought to be buried under St. Peter’s Basilica in the Vatican in Rome.
Andrew is thought to be buried at the Basilica of St. Andrew in Patras, Greece.
James (son of Zebedee) is thought to be buried at Santiago de Compostela, in Spain.
John at Ephesus, in Turkey.
Thomas in Santhome Basilica in Chennai, India.
Philip in St. Philip Martyrium, Hierapolis, Turkey.
Bartholomew in St. Bartholomew Monastery near Baskale, Turkey.
How much evidence there is for these claims being accurate, I don’t know.
I can find no evidence that more than 1 of the original 12 disciples is thought to be buried in Spain. Of course, a broader definition of "apostles" might find many more Spanish burials.
This sounds more like a comment on religious artifacts being forged rather than belief. If all the wood was collected together from medieval churches claiming to have a piece of the "true cross" you would have a 50 meter tall behemoth. Same way there are multiple "lance of loginus" around the world. St. Peter is said to have been crucified upside down in Rome, and where his remains were thrown outside the city a shrine was built, which was then turned into a church, that then became the Vatican. St. Thomas's skull is in a church on the Greek island of Patmos where it has been guided but you can only see it on certain holidays and if Greek Orthodox. Patmos is also where St. John wrote Revelations and his cave is part of a church though I don't believe they have his body. So Spain claiming to have a piece of the apostles and then some sounds like Borgia propaganda.