I have kept a few of my hubby's old shirts to use. We are going to a Epic Jumble Sale next month and in the main Church hall everything is 25 pence so I will be looking out for Striped Shirts mainly (and anything else that takes my eye!). I love looking for reusable fabrics tho I have also been lucky enough to find a few bin bags full of Cotton Fabrics for 25 pence a bag. We don't have a car and it's 2 bus rides away (about 1hr 45mins travel) with 2 kids and me on a walking stick BUT it's worth it. Last trip I got 3 Vintage Pure Wool Blankets (1940's. I collect them, My daughter steals them away to her bedroom
). Turns out 2 where made just up the road from where we now live. Hubby books the day off work as he's found some amazing things at silly prices and he loves bargain hunting. We give the kids £5 each and they spend wisely as they know they can keep the change.
I suppose the amount of shirts needed would also depend on the size and style of shirt aswell. Some being slim fit some loose fit. At that price tho I'd just get all that I liked the colour/patterns on. I'm sure they'd all find their way into projects.