Turning points exist at every scale. A scene has them, a sequence has them, an act has them. What identifies one is that the pursuit after it is different from the pursuit before it — not harder, different. Escalation is not a turn; if the protagonist is doing the same thing against greater resistance, the story is getting louder rather than moving.
The practical use is in mapping. Marking the turns on a timeline shows the distribution immediately, and the two common pathologies are visible at a glance: everything bunched in the last quarter, or a long flat stretch in the second act with no turn in it at all.
A turn needs a decision behind it to feel like character rather than plot. Something happening to the protagonist redirects the story; the protagonist choosing in response is what makes the redirection feel like theirs.
What is a turning point in a story?
A moment that changes the direction of the pursuit, not just its difficulty. If the protagonist is doing the same thing against more resistance, that is escalation rather than a turn.
How many turning points should a story have?
More than most drafts have, and distributed rather than bunched. Map them on a timeline: a long flat stretch in the second act is the most common finding and the most fixable.