Are cells from only that one cell that we are at fertilization?

No, those 2 original cells are more like a blueprint. The chromosomes contain the DNA of both of your parents, and when combined make a completely new DNA string. Imprinted on that string are the instructions for making every single cell in your body. You have bone cells, neural cells, blood cells, and many others. However, each of those cells has a complete DNA code in it. However, you do have those original cells in your body. You just have millions more of them. They are called stromal cells or stem cells. They are stored in your bones for when you bones break. Stromal cells are pretty amazing. Since they are literally the same type of cell as you were at conception, they are capable of turning into any of the cells in your body. All they have to do is make contact with one of those other cells and they immediately start to replicate and create thousands more of that cell. This is why they stay in your bones because your other cells copy themselves very well. Bone cells don't, so the stromal cells help the bones to rebuild themselves faster.