SOLVED! Using toString(16)
for the JSON file names, instead of normal numeric names
0000000000000000000000000000000000000000000000000000000000000001.json (1)
0000000000000000000000000000000000000000000000000000000000000002.json (2)
0000000000000000000000000000000000000000000000000000000000000003.json (3)
0000000000000000000000000000000000000000000000000000000000000004.json (4)
0000000000000000000000000000000000000000000000000000000000000005.json (5)
0000000000000000000000000000000000000000000000000000000000000006.json (6)
0000000000000000000000000000000000000000000000000000000000000007.json (7)
0000000000000000000000000000000000000000000000000000000000000008.json (8)
0000000000000000000000000000000000000000000000000000000000000009.json (9)
000000000000000000000000000000000000000000000000000000000000000a.json (10)
000000000000000000000000000000000000000000000000000000000000000b.json (11)
000000000000000000000000000000000000000000000000000000000000000c.json (12)
000000000000000000000000000000000000000000000000000000000000000d.json (13)
000000000000000000000000000000000000000000000000000000000000000e.json (14)
000000000000000000000000000000000000000000000000000000000000000f.json (15)
0000000000000000000000000000000000000000000000000000000000000010.json (16)
0000000000000000000000000000000000000000000000000000000000000011.json (17)
0000000000000000000000000000000000000000000000000000000000000012.json (18)
...
00000000000000000000000000000000000000000000000000000000000003e7.json (999)
00000000000000000000000000000000000000000000000000000000000003e8.json (1000)
The problem is that I started following a tutorial in which the guy use normal numbers (because he only create 10 NFTs!), the moralis-mutants video. But then I have continued to watch other videos and it seems that they must be hex file names.