Hi,
In fact it's a weired situation. Semester indicates 2 periods (trimester defines 3 periods). But many universities decided to add a "summer semester" so in fact the year is devided to 3 periods and should have been defined as 3 semesters..... (trimesters !) but they prefer to say that they have 2 semester plus a summer semester..... As if they don't want to say "triplet" and say we have "twins + 1)..... weired.....
As per your question: There should be only 2 semester (as the word "semester" indicate....) in 12 months, but in many cases you'll find out that there are 3... Told you it is weired.....
Best regards,