I went through a period of making placements, typically just using prequilted fabric and putting on a binding. To be honest, I used a favorite oval placement of mine as a pattern. I've found many of my bought/gifted rectangular placements to be too small for me, can barely put flat ware on either side of the plates. But heck, hubby and I just formally ate together at our dining room table for the first time last week or so... we've been in the new house for about 6 months. Still, I do enjoy a nicely set table every now and then.
So enough history/backstory -- I'd say you can get about 4-6 placements per yard with some trim leftover, roughly 12x18"
Here's a website link:
https://www.chairish.com/blog/what-size-is-a-placemat/