I've read that the internal frequency for PCA could be set to Fosc/6 or Fosc/2 which I suppose means Fcrystal/6 or /2...
since
1) to get 32 bits T2 is the most significant
2) T2 can run at Fosc/6
3) in 6 clock mode Fmax is 20Mhz
you can deduct that T2 can handle ~3MHz at the exte3rnal input. OK let us assume that it has to synchronize in some way and say 2MHz.
so your input is to T2ex, the PCA is driven from T2 overflow.

Erik