opencv_createsamples don't generate enough positive samples

I'm trying to generate positive samples for haar training using opencv_createsamples with one positive image and 6 background images. When I specify number of samples to be generated using -num it's only work if number of samples is less than 6. If i specify higher number like 10 or 20 it only generate 6 samples. To make more samples do i need to use more background images?