An ambiguous metaphor for plenty, since it is entirely subjective whether you consider hope to be plentiful or not.  In other words, this is only a relative plenty.

See other hope related metaphors and similes, such as Elizabeth Gaskell, George Eliot or Vasily Grossman, and Nikos Kazantzakis’ surprising conclusion as to where it begins.

‘More plentiful than hope’. 

Source: ‘The Discharge’, George Herbert, quoted in John Drury, Music at Midnight: The Life and Poetry of George Herbert (London: Penguin Books, 2014), p. 351

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