What does McCullough mean when he says, “a few more such victories would surely spell ruin for the victors”?

McCullough is referring to the number of British casualites, which could not be sustained without decimating the army itself.

Bunker Hill was proclaimed a British victory, which technically it was. But in plain truth His Majesty’s forces, led by General Howe, had suffered more than 1,000 casualties in an appalling slaughter before gaining the high ground. As was observed acidly in both London and Boston, a few more such victories would surely spell ruin for the victors.