Auto Number/System

@Hana Attia - I would recommend avoiding the auto-Number/System column property. You can solve this with formulas, but the complexity of the formulas depends on A) whether you need to dynamically move rows up and down, and then B) whether you want those unique row IDs to remaining static after they are applied.

Here's a good start to setting up helper columns behind the scenes: https://community.smartsheet.com/discussion/6757/formula-to-determine-row-number

Here's a quick start to a formula to begin populating at row X:

Alternatively, you could consider an automation that pastes the value of a hidden cell into your next row. The hidden cell would be a formula to calculate the next largest value in your series.

I hope that is helpful to you