I've head one Finnish singer said "Thank you very many" at his concert. So, is it an acceptable form?
And what about the "Thank you so many"?
Both phrases you quoted are not used by native speakers. We will always use "Thank you very much."
When we say "Thank you", we are expressing appreciation for someone. This is an uncountable noun and we do not quantify appreciation using "very many" and "so many".
Hope this helps.