"twice as many as":
"..., twice as many as ..." is an APPOSITIVE modifier. Appositives must modify some noun that comes immediately before the comma.
NOTE:
If you have an appositive modifier as an ABSTRACT NOUN - such as "strategy", "figure", "statistic", "findings", "situation", "change", "difference" etc. - then such an appositive may be allowed to describe the entire situation described in the preceding clause.
Examples:
"... rose sharply in 1990, twice as many as ... " -- WRONG!!
"... rose sharply in 1990, doubling the increase of the previous year." -- RIGHT!!
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