It depends on the particular PIC, since some PIC's can execute one instruction per cycle, and others for example, execute one instruction for every four cycles of the system clock.
Although the OP asked for info re a 20 MHz PIC16, since that has already been addressed, I am showing information for the fastest version in all the different families of PIC's. Where there are significant differences between parts in various subfamilies, like the PIC24F/PIC24EP, and PIC32MX/PIC32MZ, I am showing both separately.
I got the numbers by going onto the Digi-Key website, looking up parts for each family, and then selecting the highest speed. I then pulled up a datasheet on an example part, which also verified the MIPS value.
Family Clock Speed I/O toggle time
PIC10F 16 MHz 4 MIPS 250 ns
PIC12F 20 MHz 5 MIPS 200 ns
PIC16F 20 MHz 5 MIPS 200 ns <--- example in the original question
PIC16F 48 MHz 12 MIPS 83 ns
PIC18F 64 MHz 16 MIPS 62 ns
PIC24F 32 MHz 16 MIPS 62 ns
PIC24EP 70 MHz 70 MIPS 28 ns
dsPIC30 40 MHZ 30 MIPS 33 ns
dsPIC33EP 70 MHz 70 MIPS 28 ns
PIC32MX 100 MHz 100 MIPS 10 ns
PIC32MZ 200 MHz 200 MIPS 5 ns
If anyone has any corrections to make to this table, please don't hesitate to edit it.
MIPS is million instructions per second. I/O toggle time is the amount of time the I/O pin would be either on or off in nanoseconds (ns), and is computed as one million divided by the MIPS number.
All of these processors have the ability to turn an I/O pin on or off in a single instruction. The instructions themselves varies per processor.