Aggregate means that the sampling rate that is specified will be shared through all the channels. In other words, the sampling rate shows the amount of samples that can be sent from the ADC to the bus, which must be shared depending on the amount of active channels.
The easiest way to know if a National Instruments sampling rate is aggregate is if the specification is written as S/s (samples per second), if you read S/s/ch (samples per second per channel), it means that the speed specified is individual for each channel - usually meaning the card has one ADC per channel. Learn more about this difference here: Multiplexed Versus Simultaneous Sampling
You are correct, when sampling 16 channels:
the sampling rate will be 75 Hz aggregate: 4.68 Hz each channel
One thing to recheck is that all thermocouples are connected: + white - red