Iif i understood, you have 1200 categories, and 81 data information in each category, split in folders.
If you what to get the categories, the best way i to locate ALL the category folders in 1 main folder, for example "/training/" folder, i that way you can get all the Folders labels automatically using this code line:
location = '*path*/training';
imds = imageDatastore(location,'IncludeSubfolders',1,'LabelSource','foldernames');
*path is the complet address to get the training folder. It will give you all the data directions and labels writen en each folder.
To enter it to the CNN, you can use this other part:
trainingDS = imds;
trainingDS.Labels = categorical(trainingDS.Labels);
So you get a imagedatastore variable with "categories", and finally, enter it to the network with your defined layers and options:
[net, info] = trainNetwork(trainingDS, layers, opts);
Hope it helps, and thanks if this answer is accepted.
Any question, feel free to ask.