Before 1971, the British pound was divided into 20 shillings, and a shilling was divided into 12 pence (the plural of pennies). "Shilling" was abbreviated "s" (for solidus, plural solidi, a Roman coin), and "pence" was abbreviated "d" (for denarius, plura denarii, another Roman coin). In 1971, Britain introduced decimalisation, dividing the pound into 100 pence, and the abbreviation for "pence" became "p". The abbreviations s/d/p always indicated an amount of money, whether 1d or 1p (one penny) or 50d or 50p (fifty pence), with the abbreviations serving for amounts of one unit or multiple.
There's no abbreviation for the individual coins. If you have nine coins worth 450p, you'll have to say "nine 50p coins" or "nine 50p pieces." In the same way, "50 qts" is an amount of liquid, but "50 qtss" isn't an abbreviation for fifty quart jars. The apostrophe is already taken for the possessive:
I bought 50p's worth of penny candy.
You can pronounce the the letter "p" or write out its sound to indicate the coin. In his book Man on the Run, Paul McCartney in the 1970s, author Tom Doyle describes one of the first concerts given by McCartney's band Wings. It was at a Nottingham university with an admission price of 50 pence. The band split up and pocketed the coins after the gig, weighing their pants down. McCartney noted:
We were like children. But it was all 50 pees. You walked out of
the university and your trousers were tripping you.