There a thousands of claimed apparitions of Mary but only 17 of them have been approved by the Catholic church,
Actually that’s not quite correct.
Prior to the Church instituting processes for official approvals of Marian apparitions, which didn’t happen until the start of the 1700s, there were hundreds, maybe thousands, of Marian apparitions that are considered “traditionally approved”. The apparition was generally evaluated by the local parish or community, and if it was believed to be genuine, usually a church or a shrine would be built on the site and people would go there to pray. The one you mention, Our Lady of Las Lajas, is in this category of “traditionally approved” apparitions. There have also been some cases after the 1700s of apparitions where an investigation could not be conducted for some reason (such as the apparition was in China or Vietnam during times not friendly to the Church) but the Church has still given them full approval.
As I said, there are thousands of traditionally approved Marian apparitions, but the Miracle Hunter website has highlighted 19 of those as having special significance and the full approval of the Church.
Now turning to the apparitions that occurred after 1700 and which were subject to the investigation/ approval process, the local bishop is usually the one who decides whether to approve an apparition or not. In rare cases and for the most significant apparitions, the Vatican will also recognize an apparition that the local Bishop has already approved. Bishops can also give an apparition “approval for faith expression” which means that there is no judgment made about the apparition itself or its messages, but Catholics are permitted to practice devotions associated with the apparition, such as saying particular prayers, wearing a medal, etc.
The number of approved apparitions in each category, with no overlap between categories, is as follows:
Vatican recognized: 16
Local bishop approved: 10
Approved for faith expression: 24
Garabandal is not in any of the above categories. It remains an open file with no approval of any sort.