The property of life that involves an increase in size and number of cells is growth.

Here's why:

* Growth refers to the increase in size and complexity of an organism. This happens primarily through cell division (increasing the number of cells) and cell enlargement (increasing the size of individual cells).

* Development is a broader term that encompasses growth, but also includes changes in an organism's form and function over its lifetime.

So, while growth is a component of development, it is specifically focused on the increase in size and number of cells.