As we all know that Nico Robin’s Hana Hana no Mi is one of the most powerful and efficient Paramecia Devil Fruits. The fruit’s major strength, as demonstrated by Nico Robin, is the ability to sprout multiple body parts on any surface including the user’s own body.
With this ability, the user can take on many attackers by using many sprouted limbs. Since the user is practically the only one who knows where their limbs will sprout out, they have the element of surprise on their side, meaning that unless an opponent is incapable of being held, e.g. through Logia Devil Fruit powers, any opponent that faces the user is practically defenseless against them, making this fruit one of the most powerful of the Paramecia class.
But once, Oda (jokingly) stated that Robin could sprout “200 Hana Hana”. Many fans speculated that because of this joke was revealed pre-timeskip, pre-timeskip Robin could only sprout max 200 hana-hana and it doesn’t apply to current Robin. But in my speculation, it didn’t mean like that.
Hana-Hana referred to the length of an arm. Robin’s height is 188 cm, her arm length is approximately 100 centimeters / 1 meter. So to calculate simply : 100 x 200 = 20.000. 200 meters. That’s the radius of Robin’s power. To put simply and not making her overpowered, the radius is 100 meters in round-shape radius around her vicinity.
So, as long as she has her radius, she can apply as many limbs as she wishes in “200 hana-hana away.” This speculation is further emphasized by Buggy’s 200 bara-bara. In radius of Buggy’s legs on land, he can separate and levitate his body in exact radius for his power to be effective.
So it means although that Robin can sprout limbs in designated area as she wishes, she cannot sprout her limbs outside 200 hana-hana radius.
So, as long as it’s within the radius of 200 Hana-Hana, Robin can sprout even thousands of limbs and her entire body in the Hana-Hana radius. Not to mention that Robin needs to know the object / place her body need to be sprouted; as it was once with the South Bird where she needs to see one to catch a South Bird; because if she doesn’t know the exact object where she will sprout her limbs, she can’t sprout her body. That’s why Robin can probably have photographic memory as well, because when she sprouted half of her body to appear beside Law, she needed to memorized the Green Bit area.
And of course it explains Robin’s “limitations”, because if she can sprout her limbs on any surface as she wants without the limited radius, she will be overpowered. This 200 Hana-Hana is another “weakness” of Robin’s power to make her less overpowered, as “power and speed mean nothing to her.”
And there is a question why Robin can sprout her limbs on organic matter (like on human body), but she can’t sprout her limbs inside an enemy’s body. The logical reason is probably because as Robin’s power is Hana-Hana, an enemy’s inside body doesn’t have enough room for her limbs to grow. Another reason is because although at first Robin’s specialty is assassination, after she joins the Straw Hat Pirates, she omitted assassination as her specialty probably because it is considered too brutal and against the crew’s moral code to kill someone.