Apparently the sizing is good and old fashioned i.e. small really is small (women's size 6-8). There's a sizing chart for each garment near the bottom of the page which gives dimensions across the chest and from neck to hem so you can measure your favourite T and compare if you want.
You can order as few or as many items as you wish, though as postage gets significantly cheaper for multiple purchases it’s worth getting a few of you together or buying more than one item at a time.