When I started my first rig I used an asus p7p55d le with 4 slots PCIe and all worked well.
I never tried to put more GPUs, but I can say to you after some experience:
1 - better to use older motherboards with their limit, so in your case 5 GPUs
2 - if your motherboard supports 5 GPUs, it's probably a good one, and there's a good chance to working with 6 GPUs at the same time with an adaptor, so it's your decision
3 - if it's an Asus, more chances to work with 6, I used MSI, Gigabyte, biostar and Asus, and after all, Asus is the easiest one to work with more gpus
4 - you can research the exact model of motherboard on google/youtube/reddit + 6 gpus and see if anyone already tried, it's hard to find something because it's an old model, but there's a chance to find
And least, you didn't say which motherboard you have, it's harder to guess