1 = 1 + N

What constitutes one individual, for purposes of studying evolution?

Sean Rice, an associate professor of ecology and evolutionary and biology at Yale University, [said], "Levels at which selection acts is not a philosophical question, but is a property of the biological system. You can't correctly represent evolution if you don't recognize the right level of selection."

What? You can't just count heads?

Only for things that have them.

For unicellular life, clearly cells are individuals. But is a eukaryotic cell a part of a larger system, an individual entity, or a community amassed from endosymbionts acquired long ago?

The first approach reminds me of the tagline: "Life is anything that dies when you stomp on it."

The Latin root of individual is indivisible. You can't chop an individual organism into parts and still maintain the function of the individual.

Part of the problem is, there's a range of conditions between separate, independent individuals through various degrees of symbiosis, to effective fusion into a single organism. Mitochondria are believed to have started out as independent bacteria that were absorbed by early cells. (The first stages, going from independent existence to symbiosis where each is essential to the other's function, has been duplicated in the lab.) Now they're part of the cell.

Where, along this continuum, do you quit counting two (or more) individuals, and start calling it one more complex individual?

The existence of such continuua go a long way toward showing how to bridge the gulfs pointed out by creationists in their arguments against evolution.