Generally speaking, yes, one notice per alleged copyright infringement. But there is also the angle of timing (so 3 alleged infringements close to each other could lead to only one notice). Plus, notices are per rights holder or their agent, so again, theoretically, 3 alleged copyright infringement could lead to 3 separate first notices from different rights holders.

And combinations of the above.

See /information/law-basics about multiple infringements.