Stallion Shares - quick question

Previously I know that they used to do them based on 40 shares, being a cover for every share.

But as simple as that is I'm sure it was more complicated than that......

My question is... how are the stallion shares divided now? are they still divided into an amount of covers? and if they are how can some sire cover 200 odd mares in the year? Who owns the additional shares that are being used for the cover?

And as an owner of covering shares, I'd be very cross if there could be an additional 100 foals produced for sale at  the whim of a stallion master!!