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 “by mistake” 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 it’s highly unlikely you’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.