How many cast on stitches for a baby blanket?

I'm new to knitting...I want to make a baby blanket...about the size of a regular baby blanket. How do you know the measurements by the amount of cast on? I guess how many cast on stitched would I use to make this baby blanket? Size 10 1/2 needles...basic yarn...
So how many cast on for 36x36? The yarn is about average...
 
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