To form a Hydrogen bond, a hydrogen bond donor and a hydrogen bond acceptor are required. The hydrogen bond donors are lone pairs of electrons and hydrogen bond acceptors are hydrogens in H2O and NH3. NH3 has three hydrogens to act as an acceptor of hydrogen bond but only one lone pair of electrons, therefore, these lone pair of electrons acts as a limiting factor and ammonia can only form one hydrogen bond. However, water has two hydrogen bond acceptors and two hydrogen bond donors (two lone pairs of electrons on oxygen) which enables it to form two hydrogen bonds.
