PSYCAS letters of reference

[elicit]

I'm applying to at least two, potentially more Clinical Psychology programs through PSYCAS.  Does anyone know if, when I request my references through PSYCAS, they will have to submit one letter that will be then sent to each program, or if they will have to submit a letter per each school applied to?

Asking because it will potentially impact how many schools I apply to.  I don't particularly want to burden my references with having to go through 15 different reference letter links through PSYCAS if I decide to be ambitious and apply to that many - but if they only need to submit one, then it's just more work for me and not them.