Problems using Yahoo's API to retrive reports

We use Yahoo's API to manage more than 10 accounts and we need to download the cost data for them daily. We wrote a multi-threaded program to retrieve the cost data with report type BasicReportType.AdKeywordSummaryByDay. Initially, we used 10 threads to download the reports, but we were getting "Report ID Not found" errors all the times. After contacting Yahoo's tech support, we learnt that we can only use a maximum of 5 concurrent threads to retrieve reports since Yahoo's system will only store up to 5 reports per master account id per user. Hope this helps.