The basic is all you need to start. 6x12 and 6x24 is what I use the most.
I have a shape cut but have only used it once. If you do get one get the one that is long enough to cut a full width of fabric as it comes off the bolt. You can now get one that only cuts 2 1/2 inch widths.
I have collected many rulers over time with Joanns coupons.
Take your time and get them on sale.
6 1/2, 8 1/2 and 12 1/2 are great for squaring up sqs.
But the best ruler I have is the triangle square up ruler by Quilt in a Day. I use it for Half sq triangles to make sure they are perfect. HST's are the block you will make the most.