Why is 十面埋伏 related to the number 10?
KEY defines:
十面
as:
(on) all sides, (from) all sides

Also from a Baidu Zhidao question:
十面指的东,南,西,北，东北，东南，西北，西南,上，下 

Ten sides: east, south, west, north, north-east, south-east, north-west, south-west, above, below