what percentage of wars have been fought because of religion - how
many religous wars have there been?

This article might interest you:

http://news.bbc.co.uk/2/hi/programmes/wtwtgod/3513709.stm

Do you want to count wars before 1900?  Your question supposes wars
have a single cause. Two or three approximately equal causes is
typical plus a dozen or so minor causes, one of which is usually
religion. While religion is often an excuse for war, it is rarely the
dominent cause in my opinion. So pick a number and try to justify that
number. 4% of all wars were religious wars. Also the definition of war
has some baring: were there a million wars in the 20th century or only
13 wars? Do you want to average the Watts riots in Los Angeles equally
with World war 2?   Neil

The answer to your question is: none. 

There is no such thing, and there was no such thing as wars fought
"because of religion". Religion is always combined with other
elements, social and political.

Christian Smith writes in the introduction to the book "Disruptive
Religion" (Routledge 1998):
"[...]The conflict in Northern Ireland, for example, is not merely a
controversy about Catholic versus Protestant beliefs and Protestant
beliefs and practices. It is also a struggle over the political rule
of a geographic territory. The same is true of the "Religious Wars",
fought between European catholics and Protestants in the sixteenth and
seventeenth centuries.[...]" (p. 8).