When looking into this, the closest I got to an answer is that the Masoretes were great copyists, but that they also had a way of disposing of older manuscripts, through the process of burial, and that this is the reason there supposedly aren't many preserved manuscripts of the Old Testament.
Wikipedia lists that there are about 2000 manuscripts of the Septuagint version of the Old Testament today.
I also came across the library of a man named Abraham Firkovich, who apparently managed to obtain at least 1.300 Samaritan manuscripts of the Pentateuch, the first 5 books of the Bible. His library is called the second Firkovich collection, and it contains many Biblical manuscripts, but I can't make out whether all 17.000 manuscripts mentioned are Biblical or how many of them are Old Testament manuscripts.
Another collection is the Cairo Geniza collection, which consisted of 300.000 manuscripts before being dispersed. Cambridge holds nearly 200.000 of them in present day, but it also remains unclear how many of them are Old Testament manuscripts.
Any help or answer would be greatly appreciated.