Ok so you have 15 classes, each of these classes have corresponding numbers which indicates the corresponding images like:
class 1 : image 1, 10 , 100, 256, 3500 etc
class 2: image 2, 3
…
class 15: image 5, 240 etc
Am i correct ?
In this case, have you checked how many images of class A do you have in both training and validation set? You should have the same ratio between them, If in training set you have 1000 images of class A (which represent 1/10 of the training set as you have ~10000 images within, you should have ~250 images of class A in the validation set because you have ~2500 images within)