Methylene blue number (MBN) is defined as the maximum amount of dye adsorbed by 1 g of adsorbent.
Please, check the following paper:
F. Raposo, , M.A. De La Rubia, R. Borja, Methylene blue number as useful indicator to evaluate the adsorptive capacity of granular activated carbon in batch mode: Influence of adsorbate/adsorbent mass ratio and particle size, Journal of Hazardous Materials, Volume 165, Issues 1–3, 15 June 2009, Pages 291–299