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Both of them are right
One divided by the incidence (reciprocal) will give you the number needed to reach out in order to prevent the occurrence of disease. For example if the incidence of influenza in certain population is 5 in 1000 per year then you need to vaccinate (1000/5 = 200) people in order to prevent one influenza case per year.
However, that's not the most common way NNT is used in medical journals. It's usually the reciprocal of ARR. Which basically means how many cases you have to treat in order for the new intervention/therapeutic measure/drug ...etc can prevent/improve one case in the population of the study. For example, if the incidence of DM retinopathy is 20% in people treated with OHA and 15% in people treated with glitazones then you have prevented 5% (which is your ARR) and so you need to treat 20 patients to achieve that difference.

Please note that NNT should always be a sound number. If your calculation ended up 5.9 then you have to round it up to 6 because you cannot treat 5.9 individuals :p
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