I assume the question is 'how many Legendary Pokemon are in the Sea Spirit's Den?'
If so, there are three legendary Pokemon, specifically the Legendary birds, Zapdos, Moltres and Articuno. However, you will only have the ability to find one of them depending on your starter.
If you chose Chespin, you get Articuno.
If you chose Fennekin, you get Zapdos.
If you chose Froakie, you get Moltres.
And they are not their immediately, but you have to find them several times around Kalos, as they are roaming the region. They will flee immediately, with you having no chance to combat. You can finally battle them at the Sea Spirit's Den, having the chance to capture it.
Also, just to make Fizz happy, there is only one Sea Spirit's Den. :P
Hope I helped. :)
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