How many LiFePO4 cells can be in parallel?
There really isn't a maximum!

You have already hit the major concerns including using the same state of charge,  using batteries that are in the same condition,  from the same manufacturer (and preferably the same lot number to get an even closer match).  You also want to make sure that you never short circuit that battery pack as it will have an incredible amount of power and can release that power really quickly.  Putting the cells in parallel also lowers the internal resistance.

Where did you read that 3 is the maximum for parallel for regular lithium ion?  I built a battery pack from 40 - 18650 lithium ion cells in parallel and use it every day.  I connected a PCB to protect against short circuit, over charge and over discharge.

It is used for relatively low current, 4 amps and less, but charges at as fast as 10 amps with no problems. 

For your project I would look at the electric bicycle group.  They probably have the most experience using larger LiFePo4 batteries.  I am sure that they have added them in parallel and can share their experience.   I agree with you that the LiFePo4 is a relatively safe chemistry. Another alternative is the lithium Manganese battery chemistry found in the Nissan Leaf.  There are videos on YouTube showing people hammering nails through the battery with no fires or explosions.  The Leaf's battery runs at the usual lithium voltage of 3.0 - 4.2, unlike the LiFePo4 which runs at a lower voltage.