A single coaster...
And no indication of who made it, where to buy it, or when it was released. It's the usual rounded-corners-square shape. It has forward-facing classic Sonic in a ring with stars in it and his name. The background is light blue, but the 'dots' are actually silouette Sonic faces that get progressivly smaller as they go downward on the shape. This gives them a 'screen-tone-dots' type of feel. You can see the Sega copyright there at the bottom.
This type of coaster isn't particularly effective. It is usually a glazed wooden item that simply gets wet from a drink and absorbs nothing. The design is ok, it is trying (and succeeding) to look like something likely from the 90s. It's a bit unusual to have an item and nothing more about it.