A beach made of pebbles is often called a "shingle" beach.
Shingled beach
Beach pebbles are smooth as they are eroded by the waves
the pebbles at the front of the beach get eroded by the water
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Lots of pebbles on the beach
pebbles
Beach = sand, pebbles etc. Sea = water.
Not many. A few hundred will do in a tiny rocky cove. The main feature of a pebble beach is not that there are many pebbles, but that there is no sand. If there is sand, it cannot be a pebble beach.
Sand is the particles, beach is the place. And, a beach may be made of pebbles.
This is to do with tide patterns and percolation. A wave's swash (wave going up the beach) is very powerful and has enough power to carry large pebbles up the beach. However on a pebble beach, a lot of the waves energy is lost filling the gaps in the pebbles. All those small spaces mean the wave loses power and its backwash (wave going back down the beach) is much weaker than its swash. Only the smaller pebbles can be pulled back down the beach. This leaves the larger pebbles further up the beach.
Breccia .
The small pebbles are more likely to fall through the gaps between bigger pebbles than the other way around.