It can be a difficult task to count the amount of individuals in a population and there are many ways to do it depending on what you are counting.

Say you are counting the number of squirrels, there is an experiment and equation that you can use. You attract squirrels with food and tag them somehow. You count how many you tag. You release them again and wait awhile. Then you put out the food again and then count the number of squirrels comes and the number that is tagged. The equation you use is:
number of squirrels in the population = (number caught the 2nd time x number of squirrels you marked the first time) / the number of tagged squirrels you caught the 2nd time.

Another way you can do it is to "quadrant" off a small section of land where the population lives and count how many are in that small section. Then multiply the number of sections there are total. So if you have a piece of land that is 25 square miles and divide that into five 5 X 5 quadrants/plots and find out that there are 100 oaks in one 5 X 5 quadrant, then you multiply 100 by 5 (because you assume that there are 100 oaks in each quadrant and there are 5 quadrants.) This works well if the population you are counting is fairly stationary (like a plant or a barnacle.)

As I said before there are lots of ways of counting, each with its disadvantages and advantages. You just need to choose the best method for the population that you are interested in counting.

A population is all of the individuals of one species in a particular area, like gray whales in the Pacific Ocean or eucalyptus trees on a campus. How we count the number of individuals depends a lot on what we're counting.

Counting the number of trees in an area isn't too difficult, especially if you have a map, but things that move are harder to count. To count whales, one method used is called mark/recapture. Whales are tagged and then researchers capture whales another day. Scientists then use a formula to estimate how many whales there are.

Communities are collections of populations, such as all of the living things in the Pacific Ocean, or all of the living things on a campus. Again, scientists have to use lots of different methods to try to identify as many of the species as they can because different species have different habits.

There are a lot more species in one area than you might think. Use a piece of string to mark off an area that's about one meter per side, then carefully try to find as many species as possible in it. It doesn't matter if you don't know their names. I think you'll be surprised at the number. I guarantee that you will also miss several species.

Scientists are trying to figure out how to count the number of species within a population. They're also arguing about what are different species of bacteria and what are just normal variations between bacteria. Nowadays scientists use DNA sequences a lot to identify the number of different species in a population. They can dig up some muck with bacteria and other microorganisms in it and find all the different DNA sequences for some gene and compare the DNA sequences to see how many different categories the DNA sequences seem to group into. Then they compare them to the DNA sequences for known microorganisms to try to count the number of species and see if there are new species. I heard talks about that research this week.

A population, by definition, is one specie.

A community can have multiple species, and the way that it is counted is by simply going to the field, setting borders, and counting everything found in the area. Naturally, things get missed, because they're transient. Estimating the true species number from a count in the field is easier said than done.