traverse through multiple form values and put some of them inside struct slices

I'm in a situation where I want to get an unknown number of input values into my Go structs, but only some of them will be placed inside a slice/array. Here's a snippet of the HTML content:

<form action="/" method="post">
...
<tbody>
    {{ range .users }}
    <tr>
        <td class="text-center">
            <img class="avatar" src="{{ .AvatarThumbnailURL }}">
            <input type="hidden" name="thumbnail" value="{{ .AvatarThumbnailURL }}">
        </td>
        <td>
            {{ .Username }}<input type="hidden" name="username" value="{{ .Username }}">
        </td>
        <td>
            {{ .Age }}<input type="hidden" name="age" value="{{ .Age }}">
        </td>
        <td>
            {{ .Email }}<input type="hidden" name="email" value="{{ .Email }}">
        </td>
    </tr>
    <input type="hidden" name="userid" value="{{ .UserID }}">
    {{end}}
</tbody>
...
{{ .csrfField }}

// submit button

</form>

Where {{ .csrfField }} will generate a hidden input. This part is working fine.

Now the problem is how do I get this data inside a struct?

Let's say I have the following struct:

type UserFormData struct {
   UserID int
   Username string
   AvatarThumbnailURL string
   Age int
   Email string
}

But in my POST function I don't know how many users will be submitted through the form. So I guess I have to do something like this:

userFormData := make([]UserFormData, len(r.PostForm)/6) 

(divided by 6 because there are AvatarThumbnailURL, Username, Age, Email, UserID and csrfField)

Then can go through the r.PostForm like this:

i := 0
j := 0
for key, values := range r.PostForm {
	switch key {
	case "userid":
		userid, _ := strconv.Atoi(values[0])
		userFormData[j].UserID = userid
	case "thumbnail":
		userFormData[j].Thumbnail = values[0]
	// the other fields
	i++
	if i%6 == 0 {
		j++
	}
}

While this work, it's extremely error prone to indexoutofrange and whenever I make a change then I have to count number of input values that will be sent with the form and update the (currently) '6' values. I have no doubt that there must be a much, much, much better way to do this, and I would greatly appreciate if someone could point me in the right direction

You can take use the fact that r.PostForm is actually a map where keys are the field names and values are an array of strings, so you can do something along these lines (I didn't test it, but the concept should work).

We iterate over all values for the "userid" field to know how many users there are. Also, you can use append to add users dynamically to the slice.

userFormData := []UserFormData{}
for i, userId := range r.PostForm["userid"] {
     user := UserFormData{
        UserID: userId,
        Thumbnail: r.PostForm["thumbnail"][i],
     }
     userFormData = append(userFormData, user)
}