Why the cruiser requirement was not a two-for-one replacement

Did forty Counties and thirty smaller cruisers replace older armoured and protected cruisers on a simple two-for-one basis?

No. The suggested preference was closer to forty of each type, driven by requirements rather than a fixed replacement ratio.

The reply first corrects the terminology: the Counties were initially called light cruisers before the later heavy-cruiser category. It then rejects a simple two-for-one replacement formula for the older fleet. The preferred force is described as forty of each of the two cruiser groups. The question's replacement arithmetic misses some of the duties the new ships were expected to meet.