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I would like to know how could I create many <input type=text /> tags with a loop in JS.

I need that loop to be linked to a first input (type=number), which tell to the loops how many input text to create.

function getP () 
 var nbP = Number(document.getElementById("nombreP").value);

 for (var i = 0; i < nbP; i++) 
 var newForm = document.createElement("input[type=text]");
 newForm.id = "form"+i
 document.body.appendChild(newForm);
 

<form method="get">
 <input type="number" name="nombrePlat" id="nombreP">
 <input type="submit" value="Envoyer" id="ok" onclick="getP()">
</form>

Direct answer to your question:

<script type="text/javascript">
function getP() 
 var nbP = +document.getElementById("nombreP").value;
 var inputContainer = document.getElementById("inutContainer");

 for (var i = 0; i < nbP; i++) 
 var newForm = document.createElement("input");
 newForm.setAttribute("type", "text");
 newForm.setAttribute("id", "form"+i);
 inputContainer.appendChild(newForm);
 inputContainer.appendChild(document.createElement("br"));
 

</script>
<form>
 <input type="number" name="nombrePlat" id="nombreP">
 <input type="button" value="Envoyer" id="ok" onclick="getP()">
 <div id="inutContainer">
</div>
</form>

BUT: this is good question to learn about Javascript and HTML, but bad to create powerfull UI. To implement modern UI in JS/HTML i am strongly recommend to learn more abou next technologies:

https://reactjs.org/ or https://angular.io/ or https://vuejs.org/

I hope it helps:

document.querySelector('#ok').addEventListener('click', getP)

function getP(event) 
 let inputsQtt = document.querySelector('input[type=number]').value
 for (let i = 0; i < inputsQtt; i++) 
 let input = document.createElement("input");
 document.body.appendChild(input);
 

<form method="get">
 <input type="number" name="nombrePlat" id="nombreP">
 <input type="button" value="Envoyer" id="ok">
</form>

There are few problems with your code
First: syntax error, you are missing 1 curly bracket } to close function.

And second and more important as you click on button it causes to submit form and refreshes the page.To solve this you just need to change type of button from submit to button.

And also you can not use "input[type=text]" to create element.You can just create an element with following code

function getP () 
 var nbP = Number(document.getElementById("nombreP").value);
 for (var i = 0; i < nbP; i++) 
 var newForm = document.createElement("input");
 newForm.id = "form"+i;
 newForm.setAttribute("type","text");
 document.body.appendChild(newForm); 
 

Here's a slightly different approach, that involves adding a wrapper container within your form.

function updateForm() 

 var parent = document.getElementById('inputs'),
 count = document.getElementById('inputCount').value 

<form method="get" name="inputForm">
 <input min="0" type="number" name="inputCount" id="inputCount">
 
 <div id="inputs">
 <!-- container for dynamic inputs -->
 </div>

</form>

<!-- Notice inputs can also be associated to form with `form` attribute -->
<input form="inputForm" type="submit" value="Make" id="ok" onclick="updateForm()">