It is difficult to precisely estimate the number of children affected by prenatal exposure to antiepileptic drugs as this data is not routinely recorded by the NHS.

However, what we do know is that in 2010 over 21,500 women aged between 20 and 39 years of age were taking sodium valproate for epilepsy and other conditions.

Research has shown that around 10% of children exposed to sodium valproate will have a major congenital malformation, 12% are likely to be diagnosed with a neurodevelopmental disorder (including autistic spectrum disorder, attention-hyperactivity disorder or dyspraxia) and as many as 39% of children will require additional support in school.

Considering the numbers of women taking sodium valproate in 2010, if they all had a child, you would expect over 2500 cases of neurodevelopmental disorder. The numbers requiring extra educational support would be over 7500. Year on year this is a remarkable number of additional cases.  All of these risks are lower when the dose of sodium valproate is below 800mg and increases in size as the dose increases.
This data is from the DINLINK data (an official way of monitoring prescriptions in England and Wales) and the prevalence of problems comes from the Prospective Study carried out by the Liverpool and Manchester Neurodevelopment Group.