In an interview with Broadcast, Steven Moffat has confirmed that at least 13 episodes will be produced and aired in 2014, meaning that Peter Capaldi's Twelfth Doctor will debut with a full series in autumn 2014. Previous rumours claimed that Series 8 will be 12 episodes long rather than 13, and Moffat has not denied this rumour yet.
Moffat said of the workload on Doctor Who:
“There’s always a point in production, usually half-way, maybe two thirds, through a series when you realise you’re juggling at least one aspect of each show at once: maybe a press launch for one, another in the edit and I might not have written the last one.
“You wake up in the night thinking about 13 emergencies, each of them equally calamitous. As a lifestyle choice, it’s questionable. One of the hardest things for me to do is work out when I’ll have time to write. A whole week can disappear when I haven’t had a moment to sit down. I was amazed recently when I got a script ready in time for the tone meeting.”An official announcement about Series 8 should be coming around the time of the commencement of filming - January 2014.
[Source: Doctor Who TV]