The kind of shoebox quilt I worked on last year had all the fabric precut in the box, all we each had to do was decide how to put the shapes together. Another round robin "friendship quilt" I have done I provided one "required" fabric and a "light solid color of your choice" request (any size or shape allowed, so it was fun to arrange the results!).
Number of fabrics is individual, as is block size requested. Yardage should guarantee that the last person has enough to make a block! This is more of a challenge when you don't get the box back until the end of the year, instead of receiving a block each month. Have fun!