Use digital pins as analog - Arduino Micro
Constants are variables that cannot change. They are constant.

In this case they are variables that have been set up in the board definition files (in pins_arduino.h in case you're interested) and are named just as they have said - A6 through A11.  This complements the existing A0 through A5 that are normally there anyway.

So you just use those names and it chooses the right pin for you:

int val = analogRead(A8);


The definitions of the pins are:

static const uint8_t A0 = 18;
static const uint8_t A1 = 19;
static const uint8_t A2 = 20;
static const uint8_t A3 = 21;
static const uint8_t A4 = 22;
static const uint8_t A5 = 23;
static const uint8_t A6 = 24;   // D4
static const uint8_t A7 = 25;   // D6
static const uint8_t A8 = 26;   // D8
static const uint8_t A9 = 27;   // D9
static const uint8_t A10 = 28;  // D10
static const uint8_t A11 = 29;  // D12


So no you cannot just use the number 8, you have to use A8 which points to pin number 26. The Arduino core then interprets that number as an analog pin (as opposed to the digital pin, which is 8) and performs an analog read on it.