I hope Kathy, that your arm is doing better. You asked about how many stitches you need to cast on for a sock yarn blanket.
I am using a 80 cm circular ( minimum 32 inch), 2.5 mm (US 1 1/2)
Depending on the width of your bed, you have to use 8 stitches per inch. In my case I made a wild guess and did cast on 500 stitches (for a double bed) It might be wise to measure your bed, before starting :-)
After casting on, you just start knitting - though not in the round - at the end of the row, you turn and knit back. When changing colours you have to mind that all the ends (to weave in later) are appearing on one side. It will take time, but it sure is worth it :-)