patchwork cotton is typically 110cms wide, so this is a the basic for my calculations
IF the fat quarters (FQ) are exactly 50cm x 55 cm and cut straight you'll still need at least 12! (so when patched up you'll have 4x3 rectangles)
width would be 4x 55 =220 but remember seam allowance, folding under the edges and you need to cut selvedge (woven edges) - you'll have to be very precise, so I don't recommend it if you are a beginner (sorry!)
so 3 rows of the above sewn together would give you the length:
3x 50 = 150, which is a bit more fabric, but same rules apply (seam, selvedge, edges)
so this is your starting point, I'd actually buy 15 FQs and assemble them this way
5x 50cm +250 cm - for width
then 3x 55 for length
if you are buying fat yards (so sizes are anything between 45cmx 45cm - 45cmx 55cm ) you might need even more!
if you are buying any other type of fabric, the fabric might be wider to start with - so FQs will also increase in size (and area!), so you'll need less
if you can't find your fabric as one piece or really want it as a patchwork project of course go for it,
but it might be easier and cheaper to try and find the same fabric as a one piece 1st
try online (Ebay) or call up some fabric shops if you know the exact item number and description.
any other questions? I'll check later, happy to help more! 