One of the most commonly heard sayings in political commentary is that, "... there aren't enough women in Parliament." Of course this begs the immediate question, what would be the right number? But that actually trivialises the underlying issue. It isn't about numbers it's about how Parliament should represent the people.
To address this, the first question that should be asked is, what are MPs? What are they..? Well they are legislators. From this simple fact should flow all manner of deductions about the make-up of Parliament, but the main one ought to be that Members of Parliament should be the best legislators what we have available. In practice the route to the House of Commons is by election, and therefore you can't just choose the best legislators (interestingly an appointed House of Lords does allow you to do this); however it would be a good aspiration to be able to say that MPs are at least drawn from the best legislators available.
On this basis we can then consider the proportion of women that there should be in the House of Commons, but we must first consider this question: "is there any evidence that women are better legislators than men?" If there is such evidence then it is absolutely clear that there are not enough female MPs. However I have neither seen nor heard of any such evidence; one's quality as a legislator clearly varies but there is no evidence that sex is a determinant factor. So the right answer to "how many woman MPs should there be?" is that there is no right answer, or rather that all answers are right. So long as MPs are good at legislating then it does not matter if they are male or female.
This brings us onto the wider question of the nature of representative democracy. In our current system each MP represents his or her constituency, where this is a geographically defined portion of the United Kingdom; crucially MPs are representatives and not delegates. Note that this definition does not allow for Parliament as a whole to be representative of the population as a whole. Nevertheless people observe that there are comparatively few women in the House of Commons and they therefore state that this makes Parliament un-representative. Unfortunately this analysis is wrong, because it ignores the constitution, and wrong-headed because it would lead to a ridiculous conclusion.
If we assume for the moment that Parliament as a whole should reflect the population as a whole (and here we see the first mistake, representing the population is one thing, reflecting the population is another), then clearly there should be many more female MPs; but the requirement is to reflect the population as a whole, not just the sex of the population. Therefore there should be a proportionate number of Muslims, Hindus, Jews; a proportionate number of homosexual men and a proportionate number of lesbians too. If Parliament is to "represent" the people in this way then it needs to be the people in microcosm: there should be a certain number of MPs under 30, a proportion over 60; most MPs should be English (and not just from English constituencies); and so on and so on.
The problem is that if this is your goal, your intent and desire, then you cannot pick and choose the aspects that are represented. So my question to those who propose this model, and therefore have an idea of how many women there should be in Parliament, is this: how many MPs should be stupid? Or perhaps more kindly, how many MPs should be of less than average intelligence (and I'm not equating this description with stupidity)? The answer to that is roughly half, and if you don't understand why, then you're probably one of them.