100000
Decibel (dB) is a logarithmic scale of power. Depending upon if you're looking for power, or root power a change in the power by a factor of 10 will be 10dB, or 20dB. Since this question is asking about "sound intensity" it's looking for a power factor, not a root-power factor (which would be the case for sound pressure). So we're looking for a 50 dB increase (100 dB - 50 dB) = 50 dB which means that we're wanting 10^5 = 100,000 times as much power. Assuming each identical voice contributes the same amount of power, you'd have 100,000 (one hundred thousand) voices talking at once for a sound intensity of 100 dB.
