You won't be given a space or word limit when writing your LSAT sample.
However, if you're interested in how far you've gone, the LSAT Writing interface provides a word count just below the box where you're composing your response to the writing prompt.
Don't get intimidated by the word count. It was placed there for informational purposes. Continue writing your essay as well-argued and well-written as you can without worrying about the length of your written response.