An ability referring to the number of creatures in your party gets a number from zero to four. Such abilities never ask which creatures are in your party, and you never have to designate specific creatures as being in your party. You can't choose to exclude creatures from this count to lower the number.
If a creature has more than one party creature type, and there are multiple ways to count that creature that could result in a different number of creatures in your party, the highest such number is used. For example, if you control a Cleric and a Cleric Wizard, the number of creatures in your party is two. You can't choose to have it be just one by counting the Cleric Wizard first as a Cleric.
Once you declare that you're activating Seafloor Stalker's ability, no player can take actions to try and change the number of creatures in your party and change the activation cost.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Cristi Balanescu
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To determine "the number of creatures in your party," check whether you control a Cleric, whether you control a Rogue, whether you control a Warrior, and whether you control a Wizard. The number is the total number of those checks to which you answered yes. Each creature you control can be counted for only one of those checks.
You can activate Seafloor Stalker's ability more than once, even if it already can't be blocked this turn.
Once a creature has blocked Seafloor Stalker, activating its ability won't cause it to become unblocked or remove that blocking creature from combat.
