How do you do inventory planning? Do you use software?

Let’s say you sell 100 units a month, demand is consistent, and it’s not seasonal for simplicities sake. How many months worth of inventory do you need at Amazon? And how many do you need between Amazon, AWD/3PL (if you use one), and in transit. Also, when do you reorder?

Now many businesses have a number of SKUs, product sales aren’t consistent, and there’s more moving parts. Do you use inventory planning software to help? If so, any recommendations? Thanks!