All Series 3 mixers are technically a 32 channel mixer under the hood. Each model offers a certain number physical inputs.
The SL16 features 16 analog inputs, and 16 'digital / network / SD card' inputs.**
The SL24 features 24 analog inputs, and 8 'digital / network / SD card' inputs.**
The SL32 features 32 analog inputs.
** All three mixer's inputs can be sourced from 'digital / network / SD card.'
The 16 / 24 just have the non-analog inputs available as 'digital / network / SD card' sources, so in order for the 16 and 24 channel models to have the full 32 inputs, you will need to purchase the AVB Stageboxes that will be released at a later date to have the full 32 analog inputs, sourced from the given models physical and stagebox inputs at any time.
The mixers can not be expanded by any other products other than AVB enabled devices that are tested and qualified for use. This includes USB / FireWire / Thunderbolt interfaces which have no way of communicating with the mixers outside of AVB.