Another useful rule set which was posted on the SA mailling list. This once counts the number of recipients and if greater than a threshold add a score
describe TO_TOO_MANY To: too many recipients
header   TO_TOO_MANY To =~ /(?:,[^,]{1,80}){30}/
score    TO_TOO_MANY 0.3

describe TO_WAY_TOO_MANY To: way too many recipients
header   TO_WAY_TOO_MANY To =~ /(?:,[^,]{1,80}){50}/
score    TO_WAY_TOO_MANY 0.3