Hi Kristian,
Most, if not all, endurance coaches I know go with 2 strength workouts per week. The reason for this is recovery. You want to have about 72 hours of recover between strength sessions, which means that when you lay it out, you get 2/week. Of course you can do more strength work per week, this is why some people that do strength 4 days/week have ‘legs day’ and an ‘arms day’, they’re keeping the 72-hours of recovery between sessions that target the same muscle groups.
And yes, I am also referring, to the transition and base periods. Strength training often takes the form B and C priority events and specific training sessions such as uphill pack carries, for example, during the specific period.