Up to 16 can be used as sources of external interrupts. However, there are certain rules about which pins can be selected as external interrupt pins.
The figure below is an excerpt from the EFM32LG990 datasheet "GPIO Pinout Overview". Each pin belongs to both the Pin 0 to Pin 15 group and the Port A to F group. For example, the PD12 pin belongs to the Pin 12 group and the Port D group.

One external interrupt pin can be selected from each group of Pins 0 to 15. One from the Pin 0 group, one from the Pin 1 group, one from the Pin 2 group, ..., one from the Pin 15 group, for a total of 16 maximum. For example, in the Pin 5 group, only one pin can be selected from PA5, PB5, PC5, PD5, PE5, and PF5, and PA5 and PB5 cannot be used as interrupt pins at the same time.
GPIO_EXTIPSELL (for Pins 0 to 7) and GPIO_EXTIPSELH (for Pins 8 to 15) specify the pins used for external interrupts. If you want PA5 to be an interrupt pin, change EXTIPSEL5 (the bit for Pin 5) in GPIO_EXTIPSELL to 0 (Port A). (Although the registers are listed for understanding the rules, they can actually be set easily using the API.)