One voltage reference per DAC or one voltage reference for all DACs?
You're probably okay using a single reference, but you may well be able see a bit of crosstalk between channels.

Keep in mind that the reference input is not differential so any difference in ground potential (say due to current from the DAC output) will show up as a slightly different DAC output voltage (measured with respect to the local ground). With a 5V Vref, one LSB is less than 20uV. A 300 mil trace 500 mils long (1 oz copper) has a resistance of about 0.8m\$\Omega\$, so about 20mA will cause a 1 lsb shift in full scale. 

Not too bad, things get much worse when you're trying for 24+ bits.