How many of the five nitrogen atoms have a lone pair which is delocalised (part of the aromatic system) ?

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The nitrogen atoms (2), (3) and (4) are pyridine-type: each is doubly bonded in the ring and its lone pair lies in the plane of the ring, so it is localised.

The nitrogen (1) carries H and its lone pair is part of the aromatic sextet of the five-membered ring (pyrrole-type). The $- N H_{2}$ nitrogen (5) is attached to the ring carbon and its lone pair is delocalised into the ring. So 2 nitrogen atoms have a delocalised lone pair.

Answer: 2.

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