How many sessions of planks should I do a day?
Using this article as my guide, I should repeat the plank for 6 times as I can only hold it for about 6s.
However, should I do 2 or more sessions in one day? Strength is my goal, as I have little core strength.
You need to work on something that is easier and you can hold for longer time. 6 seconds is really not enough volume to really start improving. See, you are not even on the chart in the (excellent) article you are referencing, which starts at a max hold of 8s:
With a 6s max hold time, you would have to work at ~70% of that, or 4-5 secs... it is just too short time, even to practice consistently.
The solution is, not to increase the number of sets, but to pick an exercise that you can hold for at least 10-15 secs. Plank with the knees down on the floor, or plank on an inclined surface should work. So, for example, do 6 sets of 8 secs, or 5 sets of 10 secs of kneeling plank. This will give you a consistent base to improve from.
Btw, I think, that the low max hold rows in the table are intended for more advanced stuff like levers and planches. Beginners should work on easier stuff for longer time/more reps, because they can not really effectively/consistently work very close to their limits yet.
Perhaps you should consider easier variations of the plank exercise. Place your arms on something higher (say, a chair). This will shift more weight to your feet and help you hold the position for longer.
This will allow you to push yourself to hold the plank longer. Slowly progress to lower surfaces, and when regular plank becomes too easy for you, elevate your feet to make it even harder. Good luck!
It's beter to focus on increasing the time you can hold a plank, than to repeat it multiple times in one day. So in the beginning it will be 6 seconds, but push yourself everyday to hold them longer. Your muscles need time to recovery, otherwise they won't get stronger. Safe training!
The goal of the plank is to strengthen your core. As a result, the longer you can stay in the position, the stronger your core becomes.
To answer your question, you can perform as many sessions daily as time permit you; however, that shouldn't be your goal. Your goal should be to progressively increase the duration.
If 6 seconds is all you can currently do, endeavor to increase it. Try holding out for 8 seconds. Increase the duration once you've reached that goal.
The beauty of this exercise is that it can be performed almost anywhere :). So, do it as many times as you desire.
Ideally, you should be always trying to hold your plank for just a little longer each time, but if you hit a wall, especially an oddly consistent one where, e.g. at 6 seconds your muscles turn to jelly and you drop, trick your body. Find someone else to keep time, or set a timer and set it to longer than you "know" you can hold, then hold it as long as you can without trying to count the time yourself. Often, that's enough to break past the psychological barrier.
I would advise holding it to failure instead of multiple attempts at a "safer" value, but doing it once in the morning and then once in the evening isn't a bad plan. It gives you more than enough time to fully recover in between sessions.