How many Sea Urchins are there left in the world?

I would say there aren't many living, wild, seahorses left in the world's oceans, as many species are endangered. Here is an exerpt from WildInvest from a few years ago:

"Many seahorse species were added to the World Conservation Union's Red List of Threatened Animals in 1996. Both heavy exploitation and habitat degradation are contributing to declines of up to 15-70% in many seahorse populations over periods of 5-10 years. At least twenty million seahorses are traded each year - the majority used as traditional medicines. The aquarium trade also deals in live seahorses, despite the fact that they are difficult to keep in an aquarium. In this way, a vicious circle is created: aquarium owners often replace the dead ones, thereby contributing to their continuing decline. Seahorses are also traded as tacky souvenirs, incorporated in shell craft, yo-yos and other curiOS

Seahorses are found in nearshore habitats that are among the most threatened in the world. Degradation and destruction of these coastal eccosystems due to mangrove cutting, coral mining, dynamite fishing, trawling, land-derived pollution and a host of other human activities negatively affect these threatened species.

Seahorses are caught as target catch and trawl by-catch in the coastal waters of Vietnam. In recent years, there has been concern that the number of seahorses is decreasing. However, prior to the establishment of Project Seahorse - Vietnam, no fisheries data were available for seahorses."