When my company was implementing NetSuite, our implementation partner said that the purchase order quantity, item receipt quantity, and invoice quantity all have to match. Is this true? We have a deal with our manufacturer that they can product plus or minus 5% of our purchase order quantity and we will accept any quantity in that range. So then, what do we do if they produce a quantity that is anything other than the exact amount on the purchase order? What we've done so far is we change the original purchase order so that its quantity will match both the item receipt and the invoice. But I am wondering if this is really necessary? Can Netsuite handle it if we leave the original purchase order untouched but log an item receipt at a different quantity?

Yeah, you can simply under-receive and bill when necessary. You'll probably want to implement a process for closing the remaining balance on the then partially received PO. To over-receive, you'll need to enable the account preference

Allow Overage on Item Receipts

(Setup > Accounting Preferences > Order Management)

You'll also want to look into the Match Bill to Receipt and the Post Vendor Bill Variances Process