Hello! There are three commonly used rules when writing out numbers in literature.
The first one is numbers under "10" are written out as words (i.e 1-> one, 2-> two, 3-> three, ect)
The second one is if the number is representing a date, you write it in numerical form (i.e years stay in their number-forms)
The final rule is the one that applies for you is that if the number is the first word of the sentence, then you write it as a word instead of it's numerical form.
So the correct way you'd write your sentence is, "Twenty-six people posted messages to my blog in just thirty minutes."
