Re: Instance and cell
Cells are the basic gates in your design. A cell can also refer to the basic digital structures that are present and used in the Synthesis tool.
When a cell is used in a design, it is called an instance. For example, you have the final top-level module in your design where you call the lower-level module a number of times and connect them together. These are called instances of the leaf/lower-level module.
Instances are also the basic cells that are used when synthesizing your design. For example, if you design a counter and after synthesis the tool calls or uses the DFF cell in the tech library, the D-FF that is used is called an instance and the number of times the D-FF is used or called are called "number of instances". If the D-FF is used 5 times..then it has 5 instances in the design.
Hope this cleared your doubt!