It was announced the other day that in 2018 Doctor Who's format will change to 10 x 50 minute episodes (plus Xmas??).
When WHO returned in 2005 it featured 13 x 45min episodes, plus the Xmas special - 14 episodes - which lasted for the first 4 years. After that, showrunner RTD was by all accounts completely burnt out, and as such we then had only 2 stories, plus the 2 part Xmas/New Year story in 2009. The 11th Doctor started with 2 "full" seasons, however one was split with a 3 month break. The next season was also split - but this time with a 6 month break, spreading the season over 2 years - likely due to Moffat's Sherlock commitments (and likely a factor in Smith's departure as Doctor). When Capaldi stepped into the Doc's shoes, it was with 1 less episode per year for 2 seasons, before Sherlock again interrupted, with zero episodes in 2016 (except for the Xmas special). His final season also comprised 12 episodes, plus the upcoming special.
As such, although a "season" has typically comprised 13-14 episodes, on average we've only seen around 10 episodes each year.
The move to 5 minute longer episodes, with 2-3 less per season, one can only hope, is an attempt to increase the likelihood of producing Doctor Who year-in year-out, without the long absences and short seasons. Certainly, IMO, it would be better to have a "full" season every year, because I saw in my children their reduced interest after 2016's abysmal lack of new episodes, and remember my own disappointment with Tennant's fizzled last season.