It's amazing we're even having to debate this!https://www.rmt.org.uk/news/rmt-announces-27-days-of-strike-action-on-swr/
According to the RMT's own website, it's 27 days "in December" (in paragraph 1), though the page title and URL simply state "27 days". The RMT article then repeats "27 days" (with mentioning December) further down.
The instructions to members are to strike on the days listed at the bottom of the page. They are:
2nd to 11th December inclusive (10 days)
13th to 24th December inclusive (12 days)
27th December to 1st January inclusive (6 days).
Adding these up I get to the total of 28 days.
And yes, it's 28 days.