Set maxFileCount to 1 and DO NOT set the multiple attribute on your file input markup to allow ONLY ONE file to be selected OR uploaded at a time.

<!-- AUTO REPLACE AFTER EVERY 1 FILE SELECTED -->
<div class="file-loading">
    <input id="input-repl-1" name="input-repl-1" type="file">
</div>
<script>
$("#input-repl-1").fileinput({
    uploadUrl: "/file-upload-batch/2",
    autoReplace: true,
    maxFileCount: 1,
    allowedFileExtensions: ["jpg", "png", "gif"]
});
</script>

An alternative to the one file limit above, which includes an initial preview. This scenario consists a configuration that will automatically overwrite initial preview files and overwrite uploaded thumbnails to always show one file in the preview. Note: Initial preview thumbnail deletion capability has been disabled for this demo.

<!-- AUTO REPLACE AFTER EVERY 1 FILE SELECTED -->
<div class="file-loading">
    <input id="input-repl-1a" name="input-repl-1a" type="file" accept="image/*">
</div>
<script>
$("#input-repl-1a").fileinput({
    uploadUrl: "/file-upload-batch/2",
    autoReplace: true,
    overwriteInitial: true,
    showUploadedThumbs: false,
    maxFileCount: 1,
    initialPreview: [
        "<img class='kv-preview-data file-preview-image' src='https://placeimg.com/1920/1080/any/grayscale'>"
    ],
    initialCaption: 'Initial-Image.jpg',
    initialPreviewShowDelete: false,
    showRemove: false,
    showClose: false,
    layoutTemplates: {actionDelete: ''}, // disable thumbnail deletion
    allowedFileExtensions: ["jpg", "png", "gif"]
});
</script>

You can also validate multiple file uploads and auto-replace the files in preview after the maxFileCount has been reached.

<!-- AUTO REPLACE AFTER EVERY 5 FILE SELECTED -->
<div class="file-loading">
    <input id="input-repl-2" name="input-repl-2" type="file" accept="image/*" multiple>
</div>
<script>
$("#input-repl-2").fileinput({
    uploadUrl: "/file-upload-batch/2",
    autoReplace: true,
    maxFileCount: 5,
    allowedFileExtensions: ["jpg", "png", "gif"]
});
</script>