Hi Bahn
To limit the number of invoices you'd like to get you could simply look at the handles returned in the array from Debtor_GetInvoices and select the 30 highest numbers.
This is assuming that you're booking invoices sequentially though. With only the handles there's no way to limit the array by date.
If you're looking for a date interval you'd look at Invoice_FindByDateInterval, get the array and then sort from that.