If you want a good sized single I use between 180 to 200 stitches. Also, and this is me, not you, check the videos on You Tube for keeping the edges straight. Bella Coco has a lovely video where she shows you how to get a neat edge using trebles. Also, I tend to check out either temperature blankets, keeping all the colours of a similar hue together i.e. all the yellows together, all the reds and running them into each other or a colour wash which goes from very light to dark. Sorry, to ramble on but blankets are my passion. Good luck with the rainbow one. It sounds wonderful and one thing about basic double or treble is that you can pick it up anytime without having to worry about the pattern.