I need to know what fuse number is the fuel pump relay on a 2003 Ford Windstar

Need to test for voltage drop at the ground and feed sides of the relay and at the pump. At 15 years of age corrosion can cause problems.

Check this fixya question for diagrams:
http://www.fixya.com/cars/t9154097-need_2002_ford_windstar_central_junction

I'm pretty sure 2001-2003 use the same fuse box.
2003 Windstars only came with 3.8L engines
I believe you are looking for fuse #5 and relay #207

There is'nt a relay they are controlled through the multifunction switch which is likely the problem. Be sure to check all fuses first.

1999-2003 ModelsEngine Compartment fusebox - fuse #5, and relay #207

NOTE: You really need to have a diagram in front of you when looking for these specific fuse/relay locations. The Owners Manual is a good resource.
Autozone's free online Repair Guide is another source of fusebox information and illustrations.

in the underhood fuse box just beside the master cylinder the fuel pump relay location number is 50B, in the same box also check fuse # 23
A 20 Amp Mini fuse for fuel pump power

It Maybe Just The Fuel Pump Relay Is Bad Causing The Fuse To Blow I Recommend Checking The relay First

CALL YOU LOCAL FORD DEALER AND THEY CAN GIVE YOU THE CORRECT FUSE NUMBER THAT YOUR VEHICLE USES. PROBABLY THE SPADE TYPE FUSE AND VERY CHEAP TO PURCHASE.

fuse pump relay is under the hood in the main fuse box and labled fuel pump relay

The fuel pump power on the Windstar is controlled by a component called a Constant Control Relay Module (CCRM).
Its located located on the driver's side of the engine compartment.