Hi redant,
It's usually one essay called a statement of purpose, but usually you want to focus more on your research experience and interests than on your life story. However, and this is where it gets confusing, some programs (I'm not 100% sure about Anthropology) ask for two statements - a statement about your life background (usually called a personal history statement, or statement of diversity or confusingly, a personal statement) and a statement of purpose (or statement of academic purpose). So you just need to look at what the program asks for. If its one statement (even if they call it a personal statement) they want a statement of purpose about your research. If they ask for two, then one would be the personal history statement and the other would be the statement of (academic) purpose.
As for how long it should be, 1000 words is usually a good estimate for the statement of purpose. Sometimes the programs list a size limit, so you really need to check. If the program wants 2 statements, usually each would be smaller, but again, they sometimes list how long they want it so just check each program's requirements.
I hope this clears it up for you!