Yes, that's how it should be pronounced. The spelling reflects the etymology, but the pronunciation got simplified. If you pronounced it as it's written, you'd get "pjentnaśće", and that -ntn- cluster isn't easy to pronounce.
There aren't many words with such simplifications. Other numeric examples: in pięćdziesiąt, sześćdziesiąt, dziewięćdziesiąt and sześćset ć is silent.