Sea anemones can reproduce sexually and asexually. The depth at which they live in the ocean depends on which type of reproduction they use - the deeper they are, the more sexual reproduction there is, and the shallower they are, the more asexual reproduction there is.

Sexual: Both genders sometimes can produce egg and sperm but there are also defined sexes where they only produce either egg or sperm. Egg and sperm are released into the water at the same time based environmental cues, such as the moon cycle or low tide. They are united and produce free floating larvae, or planula, which then grow into sea anemones.

Asexual: They usually split to produce to identical organisms, or clones Some split longitudinally and some grow new organisms from their basal discs. They also use an asexual reproductive processed called budding where a part of the anemone breaks off and forms a new one organism. They reproduce every couple of days and their average life span is between 50-70 years.

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This depends on the kind of sea anemone (of which there are many species). I'm not a marine biologist and so don't know the details for any one species of sea anemone, but I believe it's a yearly thing, i.e. they produce eggs and sperm at a specific time of year, every year. How many eggs/sperm an individual anemone produces also depends on the size of the animal producing as well as the species, with bigger animals producing more.