How do you forecast the inventory stock level?

You need 3 input streams

1. History 3 months blocks so 4 blocks over the last 12 is perfect
2. Sales/Operations the users of inventory - any thing coming up to influence demand ?
3. Purchasing are there are big advantages to be had in block purchasing?

These are the 3 there are others like life cycle of product by it all gets a bit complicated - these are a good starting point.

we can make an analysis for our good's stock between what ever we received and sold last specific period of time .