I'm currently looking for a new mobo for my first unraid build and most people recommend ones with 6 sata ports at the most. Is this a limitation of unraid or do disk speeds start to suffer with any more than 6 ports?

I think its more of an availability / options thing. Not many motherboards have 10 (or any that I know of) 

Lots now have 6 or 8. So no reason to get one with less.

 

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The boards with 10 SATA ports tend to be pricey and complete with features not needed by unRAID (or servers in general). I know there are server boards with the LSI chipsets onboard, plus normal southbridge for 12-14 ports. That should work no problem with unRAID 5. But the only problem with using a board with more than 6 ports is making sure the required drivers are in unRAID. If you have a specific board, it should be easy to identify the chipsets used.

For the price that boards with that many ports cost it's cheaper just to buy a board and some add on card(s). You could get a board with 4 ports on board and a SASLP-MV8 for a total of 12 ports for over 50% less than it would cost to get that many ports on board. Add another SASLP-MV8 and you're still looking at a cheaper price for 20 ports than it would cost for a board with 12.

 

Not to mention if one of the controller chips built in to one of those boards fails you've got to replace the whole board. With the cards you can just replace the card.