The number in the name is the exact sequence length of the human form. It's there because earlier versions weren't.
HGH 191AA
Human growth hormone is a 191-amino-acid single-chain polypeptide produced by the anterior pituitary. Somatropin is the recombinant form with a sequence identical to the endogenous protein. The 191AA designation in product naming exists to distinguish that exact-sequence form from alternatives.
Early recombinant production yielded a variant carrying an additional N-terminal methionine, 192 amino acids rather than 191, which was sequence-different from the human protein and associated with a higher rate of antibody formation. Before recombinant production existed, growth hormone was extracted from cadaveric pituitary tissue, a practice halted in the mid-1980s after transmission of Creutzfeldt-Jakob disease was identified. The 191AA label is a direct inheritance of that history.
Growth hormone is among the most extensively studied endocrine proteins, with a clinical literature spanning deficiency states, body composition and metabolic regulation across several decades. Its signalling runs largely through hepatic IGF-1 production, which is why IGF-1 is the standard measured proxy in growth hormone research rather than growth hormone itself, whose secretion is pulsatile and difficult to sample.