Judging cruiser numbers against the threats of the time

Why did the Admiralty consider 56 armoured cruisers and 82 protected cruisers sufficient?

It judged them against contemporary threats and also had other vessels available.

The question's totals are assessed against the threats the Admiralty faced at the time. Other ship types also contributed to meeting those requirements. The answer considers the cruiser numbers sufficient in that setting, without offering a detailed force calculation.