Well there can not be an overall rule.

From a statistical/mathematical POV , having a SDR space with orthogonal patterns which accounting for noise has a sparsity of 0.05 (5% sparsity) the random chance of flipping any selected bit â€śby mistakeâ€ť is 1/20

for two bits is ~ 1/400, for three is 1/8000 and so on, theoretically 4-5 ON bits from a given pattern will give a high confidence the corresponding pattern is active.

Yet in real applications these bits represent actual features and properties for related things, e.g. the pattern for dogs will have a high overlap with the pattern for cats.

So my personal guess is itâ€™s highly unlikely youâ€™ll get a very useful implementation by simply picking a fixed threshold of ON bits to activate all possible patterns, considering the real world is made of simple and complex things and you might need a few couple bits to identify with high confidence some simple objects and many more for others.