Beyond-5G (B5G) aims to meet the growing demands of mobile traffic and expand the communication space. Considering that intelligent applications to B5G wireless communications will involve security issues regarding user data and operational data, this paper analyzes the maximum capacity of the multi-watermarking method for multimedia signal hiding as a means of alleviating the information security problem of B5G. The multi-watermarking process employs spread transform dither modulation. During the watermarking procedure, Gram–Schmidt orthogonalization is used to obtain the multiple spreading vectors. Consequently, multiple watermarks can be simultaneously embedded into the same position of a multimedia signal. Moreover, the multiple watermarks can be extracted without affecting one another during the extraction process. We analyze the effect of the size of the spreading vector on the unit maximum capacity, and consequently derive the theoretical relationship between the size of the spreading vector and the unit maximum capacity. A number of experiments are conducted to determine the optimal parameter values for maximum robustness on the premise of high capacity and good imperceptibility.