I had only known of 9 children: Branca (1388), Afonso (1390), Duarte (1391), Pedro (1392), Henrique (1394), Isabel (1397), Branca (1398), Joao (1400) and Fernando (1402). But, given the times and the some times confusing information, why wouldn't it be possible that Philippa had given birth to three other children that might have died very young?
I think it's quite plausible. Where did you get the information from, aron?