In a Ditloid, "99 D in P" stands for "99 Degrees in a Perfect Circle." Ditloids are puzzles where a phrase or saying is represented by numbers and initials, and in this case, it refers to the fact that a perfect circle has 360 degrees, but the phrase emphasizes the completeness of 99 degrees within that context.
240 Pence in a pre decimal pound
12 Picture cards in a deck
Peugeot 205
12 picture cards in a deck
4 Doors Per Car
3 Point Basket
2 Goalkeepers on a Pitch (Football)
The ditloid 88k on a p stands for 88 keys on a piano.
4 and Twenty Blackbirds Baked in a Pie
elements of the periodic table? I think.
5 Events in the Modern Pentathlon
5040 columns in a word processor.