Just got into this mining thing with NiceHash on my daily driver computer and really enjoy it. I am planning to build a rig with spare parts I have laying around, one of which is this motherboard:

https://www.newegg.com/Product/Product.aspx?Item=N82E16813157477

I plan on building an open air rig with risers and will get my GPUs whenever/wherever I can as they become available at a decent price.

1. How many GPUs can I comfortably run off this mobo with the A10-7890K CPU I already have in it?
2. Which riser cards/cables are ideal?
3. Is 80% PSU ratio the sweet spot or should I look for a bit more extra power?