Interesting question! It is really tough to determine the volumetric capacity of the 'average' human mouth. The attached link to a scientific study states the capacity as about 71 `+-` 15 milliliters (ml) for males and 55 `+-` 13 ml for females. So we can assume about 70 ml and 55 ml as the approximate volumes of average mouths in males and females, respectively. A ml of water contains about 20 drops. That means an average human male mouth can contain as many as 1,400 drops (70 ml x 20 drops per ml) of water. In comparison, a human female mouth can hold 1,100 drops (55 ml x 20 drops per ml) of water.
Seawater contains both zooplankton and phytoplankton. The concentration of planktons (I assume you are including both zooplankton and phytoplankton) will be a function of where exactly you take a sample from. Estuaries will have a high concentration as compared to a sample harvested from deep down in the bottom of the ocean. The concentration will depend on a number of factors, including the availability of food and nutrients, sunlight, etc.
Hope this helps.