I would use a filedatastore to load all the data into a single variable (doesn't matter if they are csv or xlsx). You can see an example of how to use one to do this in this video from the Data Processing with MATLAB specialization on Coursera.
Here is the final code from that example. You can modify this to work for your data.
flightsDataStore = fileDatastore("flights*.csv","ReadFcn",@importFlightsData,"UniformRead",true);
flightsAll = readall(flightsDataStore)
Once complete, all the data from all files matching the pattern "flights*.csv" are loaded into the variable flightsAll.
