High contention ratio, on a little copper cable, in other words?
There's no contention
* over your own copper wire(s) that go from your house to the DSLAM at the exchange, but it's from there that it gets tricky...
In the old days of 512mbps broadband a 'contention ratio' of 20:1 meant that for every 20 customers with a 512mbps DSL connection to that exchange there was 512mbps of bandwidth out of the exchange. If every single DSL customer was using it to capacity at the same time you'd see shockingly low speeds, but since this is very unlikely everyone saw usually OK speeds. This stopped being relevant ages ago though...
Upgrades to the exchanges (direct(-ish) connections to the Internet, or rather to your ISPs network, rather than traffic first going over BT's own ATM backhaul network) and the way that ADSL works (coupled with LLU) changed the way this all works, so it's not that simple any more. There's still going to be finite bandwidth somewhere along the chain...
* Voice lines can be multiplexed over a single pair of copper, but only one DSL line can run over a single pair of copper.