Good point. Connections make the situation even worse (for aliens). Interestingly, it's about the same for humans: 3-4 entities produce 3-6 connections. Both are in the same neighbourhood. The limitation is biological though (http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_...).
I think what Dennis means is that there is a philosophical reason why working memory is limited, and it's not just an idiosyncrasy of organic meat-brains.
Of course the specific number is highly affected by human biology.
