You have style="font-size: 10px;"
as an attribute in that input
tag in your HTML. Inline styles overwrite external CSS. Just erase that and it works as desired:
input[type="number"] {
width:5rem;
font-size:100px;
}
<div class="introData">
<label for="introData" class="label">Introducir los segundos que se mostrará cada palabra</label>
<input type="number" name="introData" id="inputData">
<button id="start" type="submit" class="btn">
start
</button>
</div>
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