My mead finished at 0.996. How much honey to sweeten 19 L up to 1.010?

Each gravity point (0.001 SG) needs about 2.6 g of sucrose per litre. Honey is only about 80% sugars by weight, so it takes roughly 3.7 g per litre per point.

Points to add: (1.010 − 0.996) × 1000 = 14 points.
Honey per litre: 14 × 3.7 = 52 g/L.
Total: 52 × 19 = just under 1 kg of honey.

Stabilize first, add the honey in stages, and taste as you go: a high-acid mead carries 1.015 the way a soft one carries 1.008, and honeys vary enough that the factor is an estimate, not a guarantee.