@if(Model.comments != null)
{
    int i = 0;
    foreach(var item in Model.comments)
    {
        if(i<10)
        {
            <div class="prvMsgs">@MvcHtmlString.Create(item.text)</div>
        }
        else
        {
            <div class="prvMsgs rest">@MvcHtmlString.Create(item.text)</div>
        }
        ++i;
    }
}
<div id="showAllComments">Pokaż wszystkie</div>

<script>
    $(document).ready(function () {
        $('.rest').hide()
    });

    $('#showAllComments').click(function () {
        $('.rest').show()
    })
</script>