Creating unique ID based on the number of entries

Hey all,

I am looking for a formula that will select a unique ID from a table of possible IDs based on the number of entries in multiple pages. In the attached document, the formula would go in the green highlighted cells on sheet 1, 2, and 3 (B4:B15). The formula would assign one of the IDs from the table on the worksheet ID_Database to the cells that have a value in them.

For example, on sheet 1 cells B4:B7 would be 1,2,3,4 then on sheet 2 cells B4:B7 would be 5,6,7,8,9. Wherever the next name is added (sheet 1,2, or 3 would get the value 13 given there are already 12 names)

I'm not sure if I need a macro for this, but I would like to solve it with a formula if at all possible.

Cheers