in the following sentence: The same holds true for children who suffer from the disabling effects of childhood trauma. However, additional interventions targeting individual's needs will often be necessary" Do I use individual's or individuals'?
As Kate Bunting says, it's individuals' I have upvoted that answer.
Here the reason. Although the word individual sounds singular because it relates to one person, it is possible to have two or more individuals. This is the case in your sentence. You could have written
"...additional interventions targeting needs of individuals will often be necessary..."
Because you have used the plural, you must, by the normal rules, place the apostrophe after the final "s".
However, you could just as easily use "individual" in the singular, e.g.
"...additional interventions targeting needs of each individual will often be necessary..."
This would then give you
"...additional interventions targeting each individual's needs will often be necessary"
Why is this so? It is because, in your original sentence, you did not place an article before "individuals". This makes it plural. If it were singular you would have to have either the article "the" or the article "a". In my example, I used "each" instead of an article. That is also possible.
In the case in Kate Bunting's answer, Kate placed "an" in front of "individual" to show that it is singular.