Can you emboss a single layer card and should I if you can?
Embossing effects are more pronounced on thicker cardstocks.
Generally speaking, past 18-point thickness, any paper thicker is produced by mounting multiple sheets together. If you mounted two (2), 18-point sheets together, that would be considered 2-ply and would result in a thickness of 36-points. Similarly, if you mounted three (3), 18-point sheets together, that would be considered 3-ply and would result in a thickness of 48-points.
With that said, you do not want to emboss a single layered card. First off, it would most likely only be 18-points in thickness and the embossing effect will not be very pronounced. Secondly, and most importantly, when you emboss a single layered card, the back side of a card will be affected by the embossing process. Embossing will show a reverse impression on the opposite side of the stock (deboss). So if you had any text on the backside in the same area being embossed on the front, it could make for unpleasant results.
To avoid this, if you emboss a thicker 2-ply card, the backside of your card now is the bottom of the two mounted sheets (which has not been embossed at all). So the reverse impression of the embossing is no longer visible as it is hidden when the two cards are glued together to form your ultra-thick card.