<blockquote>
  <p>I'm trying to expand my neural network to also classify letters in license plate. But i'm worried if i just simply add more classes into output, for example add 10 letters into classification so total 20 classes, it would be hard for neural network to separate feature from each class.</p>
</blockquote>

<p>You're far from where it becomes problematic. ImageNet has 1000 classes and is commonly done in a single network. See the <a href="https://papers.nips.cc/paper/4824-imagenet-classification-with-deep-convolutional-neural-networks.pdf" rel="nofollow noreferrer">AlexNet paper</a>. If you want to learn more about CNNs, have a look at <a href="https://arxiv.org/pdf/1707.09725.pdf" rel="nofollow noreferrer">chapter 2 of "Analysis and Optimization of
Convolutional Neural Network Architectures"</a>. And when you're on it, see chapter 4 for hirarchical classification. You can read the summary for ... well, a summary of it.</p>
Is it better to make neural network to have hierarchical output?
<p>I'm quite new to neural network and I recently built neural network for number classification in vehicle license plate. It has 3 layers: 1 input layer for 16*24(382 neurons) number image with 150 dpi , 1 hidden layer(199 neurons) with sigmoid activation function, 1 softmax output layer(10 neurons) for each number 0 to 9. </p>

<p>I'm trying to expand my neural network to also classify letters in license plate. But I'm worried if I just simply add more classes into output, for example add 10 letters into classification so total 20 classes, it would be hard for neural network to separate feature from each class. And also, I think it might cause problem when input is one of number and neural network wrongly classifies as one of letter with biggest probability, even though sum of probabilities of all number output exceeds that.</p>

<p>So I wonder if it is possible to build hierarchical neural network in following manner:</p>

<p>There are 3 neural networks: 'Item', 'Number', 'Letter'</p>

<ol>
<li><p>'Item' neural network classifies whether input is numbers or letters.</p></li>
<li><p>If 'Item' neural network classifies input as numbers(letters), then input goes through 'Number'('Letter') neural network.</p></li>
<li><p>Return final output from Number(Letter) neural network.</p></li>
</ol>

<p>And learning mechanism for each network is below:</p>

<ol>
<li>'Item' neural network learns all images of numbers and letters. So there are 2 output.</li>
<li>'Number'('Letter') neural network learns images of only numbers(letter).</li>
</ol>

<p>Which method should I pick to have better classification? Just simply add 10 more classes or build hierarchical neural networks with method above?</p>