If I enter more than 8 items in a sales order and those items get copied into delivery doc then to billing doc.  But our client want to restrict number items to 8 (max) in an invoice.

You can restrict delivery with delivery tolerance, thereby causing invoicing of the desired.

or

Go to SPRO>>>IMG>>>Sales & Distribution>>>Billing>>>Billing Documents>>>Country-specific features>>>Maintain Maximum No. Of Billing Items.

This is where you can define a maximum number of items allowed for all billing documents within a given sales organization.

However, maintaining data here is not enough alone to bring about a billing split by number of billing items. 

You must also go to copying control for order/delivery to billing document at item level under "Data VBRK/VBRP" and maintain routine 006 "individual invoice limited" or a similar routine that accesses the data maintained here.

SAP SD Tips by : Amol

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Billing cannot be Release to Accounting

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