You could Export them using the Export button bottom right, which has the number of invoices displayed on it, and then use EXCEL to count, total or whatever with the data
if you have a consecutive numbering system for your invoices, you can easily know the total number of invoices.
also, if you enter the year in the search option, all the transactions of that particular year alone will be displayed. now the Export button will show you the number of invoices.