How many stiches to cast on?

I created a blanket and didn’t realize it wasn’t wide enough until I cast off. I am going to create another piece and attach it to the already existing piece. When I first cast on, I started with… hmm… maybe 144 stitches, plus or minus a few. I am sure that I accidentally increased a few times while knitting, which means I don’t know the correct amount of stitches right now.

It is about 56 inches long x 36 inches wide right now. I used size 10mm (actually, I’m not sure if it’s mm or US - they’re really small) circular needles. I cast on its length and knitted its width. Does that make sense?

So, my question is: how many stitches should I cast so the new piece is as long as my current piece?

If you want to make it wider, given there’s no way to accurately tell you how many to CO given the info you provided, perhaps you could consider picking up sts along one long edge and working perpendicular until you have the width you want. You could then trim the entire piece with a common, crocheted border. Something like this:

XXXXXXXXXX
XVVVV<<<<X
XVVVV<<<<X
XVVVV<<<<X
XVVVV<<<<X
XXXXXXXXXX

Picking up stitches? Does that mean knitting onto the current piece? Sorry, I’m not very term-savvy.

Go to the Tips section of this site and near the bottom of the page is a video for picking up stitches. You start at one corner of the blanket and pretend it’s a needle full of sts and pick up or make sts along the edge. Usually pick up 3 for every 4. Then knit that direction until it’s the size you want and put some kind of border around it.

Thanks!