In my room, there is one table and six chairs.
I brought back six stones from the river.
I bought six packs of cups.
Any number with a つ attached to it means you are counting that number of things. In this case, it's six things.
Since this word consists of a kanji with hiragana attached, you can guess that it uses the kun'yomi reading. Here's a mnemonic to help you do that:
What are the six things that you need to count? This time you are counting six cows (むっつ). The reason it's six, you know, is because you wanted a half a dozen cows. No more, no less.