You must be able to pay 1 iron for each Industry tile you remove during the Develop action. The rulebook states:

• "Remove 1 or 2 Industry tiles from your Player Mat, and return them to the box. Each Industry tile is removed separately and does not need to be of the same industry, but must be the lowest level tile of the chosen industry (as it is removed)."
• "Consume 1 iron for each tile removed (see 'Consuming Iron')."

There is no rule allowing you to remove more tiles than you can pay iron for. Therefore, you may only remove as many tiles as you can pay the iron cost for. If you do not have enough iron to pay for all tiles you wish to remove, you must reduce the number of tiles removed to match the iron you can pay.

Source: page 11, Develop Action section.

Source: Core Rulebook (pp. 10, 11, 6, 8, 9)