What determines how much short-term memory can hold? ScienceDaily has an item on this (here). The paper is by J.Kaminski etal., Beta band oscillations engagement in human alertness process, in International Journal of Psychophysiology.

A human being can consciously process from five to nine pieces of information simultaneously. During processing these pieces of information remain in the short-term memory. In 1995 researchers from Brandeis University in Waltham suggested that the capacity of short-term memory could depend on two bands of brain’s electric activity: theta and gamma waves. However, only now, through carefully designed experiments conducted at the Nencki Experimental Biology Institute of the Polish Academy of Sciences (Nencki Institute) in Warsaw, it was possible to unambiguously prove that such a relationship really exists…The hypothesis formulated by Lisman and Idiart in 1995 assumes that we are able to memorise as many ‘bites’ of information, as there are gamma cycles for one theta cycle. Research to date provided only indirect support for this hypothesis…Only based on discovered correlations the ratio of the length of theta wave to gamma wave was determined and the likely capacity of verbal short-term memory was determined…Following the EEG recording, the volunteers, were subjected to classic short-term memory capacity test…(They) observed that the longer the theta cycles, the more information ‘bites’ the subject was able to remember; the longer the gamma cycle, the less the subject remembered… correlation turned out to be very high and it confirmed the hypothesis of Lisman and Idiart.

So we can manipulate as many of ‘bits’ of information in short-term memory as we can fit beta waves into a single theta wave. Each ‘bit’ would be carried by a beta wave.