Add cell content into Formula command in another cell for multiple files search

Can anyone help me how to determine the correct formula for this requirement?

Assuming I have 2 individual excel files and an index excel file (in reality, there are more than hundreds of file). For index excel, once the user enter part number (eg. 1 or 2), the excel will look for the part number excel file and determine vlookup function.

The only problem I have here is I can not make the index file automatically add the part number shown on most left column into the required formula (replace the part number file section).

I tried use the indirect function but this require each file to be opened, which is not possible for actual use. I am looking for a function that can work in closed worksheet. Attached files. Please help. Thank you.

=IF(A14=1,VLOOKUP(B14,[1.xlsx]a!$A$2:$B$3,2,FALSE),IF(A14=2,VLOOKUP(B14,[2.xlsx]a!$A$2:$B$3,2,FALSE)))

Thanks for reply. But for part number column, this is not always 1 or 2 in squence, it may be 3 or 65 or 43. As I have more than 600 individuals files, it is impoissible for me write the command in this approach. That is why I was hoping to find a formula that can add whatever the number is entered in part number column and change it to file name and input into each individual formula for search. Thank you.