how many SPI slaves dsPIC33EP series MCU can drive at the same time?
SPI slaves can be selected by dedicated Slave select pin  SS  or chip select pin CS.  But, no MCU will have more than 10 chip selects I think. Hence, I propose below way of handling the 28 slaves. I have done the same for SPI flash devices and was able to upgrade firmware of all 32 SPI flash, one at a time, using one controller.  

If the command for all slaves is common, then the MOSI pins of all slaves can be shorted together and also the clock pins. The MISO pins and SS pins of all the slaves are connected to the master's MISO and SS via multiplexer. The master issues the measure command, and then sequentially selects the slave MISO and CS pins to read the data via multiplexer. Since, only one slave is connected to the master, rest all slaves will ignore the data read command.  

While issueing the measure command, the CS of all the slave devices should be low, which has to be taken care of with an add on circuit. This is required only if there is a need to issue measure command to all the slaves at the same instant.  

Shorting of multiple pins will demand slowing down the clock frequency of SPICLK due to added capacitance of many slaves. In worst case, it may be needed to buffer in two stages.

Edit: Since. CS pins aer GPIOs of MCU, they can be connected directly to all the slaves without any buffer. the SPICLK, SPIMISO an SPIMOSI needs to be buffered.   

http://www.nxp.com/documents/data_sheet/74HC4050.pdf is better suitable compared to my earlier suggestion. still, it works. 

A simple opamp used as voltage follower can be used to buffer at least 8 slave inputs.  

TLV2462 is one such example which can be used as voltage follower (buffer) for MOSI and CLK. It is a rail to rail opamp and can be used to buffer the present load. Add a small resistor at the output pin of opamp if the output seems to have ringing effect.