Files are sorted alphabetically when you add them to a transfer, and sent in that order.
How ASCII ordering works
WeTransfer follows ASCII ordering, which sorts characters in this sequence: numbers first, then uppercase letters, then lowercase letters.
This means that if you upload files with numbers in their names, they'll sort as text strings rather than numerically. For example, File-14 will appear before File-2 because the character '1' sorts before '2'.
If you want numbered files to sort in the order you'd expect (1, 2, 3 ... 14), use zero-padded numbers in your filenames — for example File-001, File-002, File-014.