Forms Personalization for count in multi record block

I want to do a personalization on the basis of the number of records selected. e.g. suppose we are doing some action on multiple invoices selected on Payables form at the same time, then i want to get the count of the invoices which are selected at run time on which action would be performed. So say suppose there are 10 invoices on Invoice form and i select three / four of them to perform some action, i want to know while doing form personalization, how to get this count of three / four.

Regards

Samarth