There are four 10 inch by 10 inch plates.
These do have slight defects in them. There may be a bit of discoloration and they are slightly bowed. It does mention this in the description on Amazon, however my kids have been able to play with them on the floor. It also mentions that they are perfect for gluing down to a table. Just add a bit of weight while the glue dries and it will work great.
As far as being Lego compatible, these work just fine. You can't really tell that they aren't Lego brand other than the minor defects. They are still a decent thickness, are easy to use and are cheaper than the Lego brand plates. These are getting tons of play from my kids and are holding up nicely. We hope to get a table to attach them to soon.
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