12

I have 2 solutions but only 1 answer.

I. 90-18=72, 72-16=56, 56-14=42, 42-12=30, 30-10=20, 20-8=12

II. 90/9=10, 72/8=9, 56/7=8, 42/6=7, 30/5=6, 20/4=5, 12/3=4, 6/2=3, 2/1=2