Regs state the 'Head of the drain' i.e. the highest point should be ventilated. I'd personally put at least one one vent on each property, drains are funny things, and need to be able to breathe.
It also avoids any issues if there were any blockages in the drains further upstream from you stopping the vent at the head of the drain from venting the other waste systems and causing problems, basically there as a fail safe.
That's why its important when considering any changes to an individual property's drains and vents and having a look at what everyone else has done. Went to an issue once where everyone on the run had put in AAV's as they all though it was a good idea to remove the 'over the gutter' vents, caused no end of problems with surges and slow draining in all 6 properties. Re-instated one vent and things were much better.
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