The ampacity of NO 12 aluminum wire with RHW insulation installed in a raceway that has 19 other wires is what?

A #12 copper conductor with an insulation factor of 90 degrees C is rated at 20 amps.

A #12 aluminium conductor with an insulation rating of 90 degrees C is rated at 15 amps.

These conductors ratings only applies to three conductors in a raceway. To apply the derate to the conductor requires the size of the raceway.

From 7 to 24 conductors in a raceway, both aluminium and copper conductor's ratings have to be reduced by .70, so 15 amps x .7 = 10.5 amps and 20 amps x .7 = 14 amps respectively.