Map Viewer Classic does not support multiline labeling. Check out Map Viewer and there is word wrapping and you can build an arcade expression to set multiline labeling.
Thanks for your suggestion. Could you please share the Expression as i m not that much good in Arcade
I've used this Arcade expression.
$feature.(yourfieldname) + TextFormatting.NewLine + $feature.(yourfieldname)
It is from this ESRI help page. https://pro.arcgis.com/en/pro-app/latest/help/mapping/text/specify-text-for-labels.htm