How many sensors should there be on a 99 F250 with the 5.4? 2, 3, 4? I seem to get different results through research.

I have the P0171 and P0174 codes and was going to replace all of them. PCV and MAF appear to be in good shape.

 

Can you put your scan tool into the monitoring mode to see which one is malfunctioning? IIRC there is a selection on some scan tools that lets you watch what is going on, and or get updates on various monitored components. Sorry I don't know who may O2 sensors you should have.
But the following article may help.
Regards,
Eric
Part 1 -How to Test the Oxygen Sensors (Ford 4.6L, 5.4L)

 

Most 5.4L have an oxygen sensor for each bank then one after the converter. If there are many miles on the sensors (2015 minus 1999= 16 years) there is always the possibility the sensors can become sluggish and even stick rich or lean. Unless you have a scan tool as Eric suggested that can read and display oxygen sensor activity and fuel trim you could wind up replacing properly working sensors. If you choose to do that your best bet is to purchase Motorcraft sensors from Ford or a Motorcraft supplier.You need to be sure to get the correct ones for your particular vehicle because certain years had different sensors. (Stick with Ford).