Congratulations - good litter size. I'm surprised that only 1 in 10 is registerable - is that usual?
Unfortunately it is a problem with OSB's - however by being very selective with what is registered and used as breeding stock the breed is slowly becoming much more uniform - meaning more and more sows are throwing litters with higher numbers of registerable weaners.
Gilts should ideally have 14 teats but 12 is fine so long as they are straight, Boars should always have 14 6 of which are in front of the sheath.
In my expiernece OSB's have ways of shocking you, a piglet which may look the best in the litter at birth often goes the other way and ends up not quite up to scratch - it's normally the one that you borderline about registering which ends up the nicest pig.
From our last litter 9 in all 2 gilts and 7 boars - neither of the 2 gilts were any good and 3 of the boars looked nice, the best looking one developed a large white blotch on his side at around 8 weeks old, the next best one ended up with a white bloch on his hind at around the same age and the one we were boarderline with has ended up the best looking pig we have ever bred and now it staying with us as a breeding boar!
OSB's are easy to keep but a pain in the butt to breed!