Regarding the maximum number of pin-driven interrupts, ScruffR’s advice still stands - there’s definitely more than 5 interrupt gpio sources - internally there are 16 channels. Not all of these are exposed to external pins.
/*
* GPIO_PinSource0: A7 (WKP), P1S0, P1S2, B2, B4
* GPIO_PinSource1: D5, RGBR, P1S1, P1S5, B3, B5
* GPIO_PinSource2: A2, RGBG, C0, PWR_UC
* GPIO_PinSource3: D4, A1, RGBB
* GPIO_PinSource4: D3, A6 (DAC/DAC1), P1S3, RESET_UC
* GPIO_PinSource5: D2, A0, A3 (DAC2)
* GPIO_PinSource6: D1, A4, B1
* GPIO_PinSource7: D0, A5, SETUP_BUTTON
* GPIO_PinSource8: B0, C5, PM_SCL_UC
* GPIO_PinSource9: TX, C4, PM_SDA_UC
* GPIO_PinSource10: RX, C3, TXD_UC
* GPIO_PinSource11: C2, RXD_UC
* GPIO_PinSource12: C1, RI_UC
* GPIO_PinSource13: D7, P1S4, CTS_UC, LOW_BAT_UC
* GPIO_PinSource14: D6, RTS_UC
* GPIO_PinSource15: D5, LVLOE_UC
*/
This table includes all pins arranged by source for the Photon, P1, and Electron, so not all of them make sense for the Photon.
I count 13 separate pin sources (including RX/TX) on the Photon.