Not so many, although you may see some after a storm. Normally though, the waves are very mild. Also, they manicure the beach and filter out things from the sand frequently in the evening so best time is to go in the early morning.
If you want to look for seashells, try going near the tidepools past Sandy Beach (east of Waikiki past Diamond Head and Hawaii Kai). Also, try Yokohama Bay on the Waianae Coast if the waves aren't rough, again after a storm is best when fresh shells wash up on the beach.
Along the North Shore there is an uncommon, tiny scallop that is a collector's item you can find on the beach, comes in different colors (red, orange, yellow, multi-colored, tec.). You will see it in jewelry where they command high prices. I have never found one and people who have are very secretive WHICH beach they found them on.
If you want to see live tropical shells, you need to go where people normally DON'T go. For instance if you want to see live cowrie shells (serpent heads for instance), go to the mouth of the Honolulu Harbor at low tide, there is a state park few go to because it is at the end of an industrial area, and on the other side of the harbor is the international airport, you will hear the planes whiz overhead. Look in non-sandy, rocky areas along the shore. Directions: From the Nimitz Highway (if coming from the west) or Ala Moana Blvd (if coming from Diamond Head/Waikiki direction) take the turnoff to Sand Island Access Road. Drive to the end to the Sand Island State Park (I think it is called Sand Island Parkway) to the parking lot. You will see the beach. This is a State Park and theoretically you are not supposed to take/kill the shells...
WARNING: If you see live cone shells, DO NOT PUT THEM IN YOUR OUTSTRETCHED PALM, they are carnivorous and have a venomous dart / sting. Some are fatally poisonous so beware! If in doubt hold the shell from the top pinched in your thumb and forefinger in the fattest part of the shell, and if the animal starts coming out DROP IT.
http://www.marinelifephotography.com/marine/mollusks/gastropods/cones/cones.htm