The compound formed by the reaction of ethanal with semicarbazide contains _______ number of nitrogen atoms.

Ethanal ($$CH_3CHO$$) reacts with semicarbazide ($$NH_2NHCONH_2$$) to form a semicarbazone through a condensation reaction.

The reaction is:

$$ CH_3CHO + NH_2NHCONH_2 \rightarrow CH_3CH=NNHCONH_2 + H_2O $$

The product is acetaldehyde semicarbazone: $$CH_3CH=N-NH-CO-NH_2$$.

Counting the nitrogen atoms in the product:

1. The $$=N-$$ nitrogen

2. The $$-NH-$$ nitrogen

3. The $$-NH_2$$ nitrogen

Total nitrogen atoms = $$3$$.

Therefore, the answer is $$\boxed{3}$$.