For Pythagoras, 10 was the symbol of the universe and it also expressed the whole of human knowledge.
At the Mayas, it represents the end of a cycle and the beginning of another. The ten was regarded as being the number of the life and the death.
In China, the cross represents the number 10 - as the totality of the numbers. Ten implies completeness of order, nothing lacking and nothing over. It signifies that the cycle is complete and that everything is in its proper order.
Ten is the first of the triangular numbers which has a center. A triangular number is one that can be arranged into a perfect equilateral triangle. Start with a single counter (coin, marble, or indeed anything, but round objects work best). Then add a row of two under it to form a triangle. Thus three is a triangular number, but has no center. Then add a row of three under that, thus six is a triangular number but also has no center. Add a row of four underneath and we have a triangle of ten, which has a center. The central counter of the "ten" triangle is the fifth. The progression of triangular numbers continues on to infinity. It turns out that every third triangular number has a central position. However ten is the only on in which the central position is occupied by the half of the total number.