That's impossible to answer. Religions come and go even today. Several indigenous tribes in remote jungles will disappear within a couple of years. With them, their religion. Other religions or newer variations of religions simply never become main stream. Those native tribal and sprouting religions with very few adherents are also religions.
Political convictions can change into religions as well. For example communism is effectively dead, as a political system. Still plenty of people who adhere to it, and refuse to debate it. That comes very close to becoming a religion. (I don't say that communism is good or bad, this is just an example). Will New Age become a mainstream religion or fade away? Too early to say. Druidism was practically eradicated by the Romans. But it fairly recently got a new life. Same for some forms of paganism. So, even religions that became extinct can come back.
In Holland we had Lou de Palingboer. Translated in English: Lou the Eel seller. He claimed to be Christ reincarnated and had some following. After his death in 1968 the movement disappeared. Since 1968 many other religions/sects/cults came and went.
With regard to the ancient Greek religion, I had a very funny experience. Long time ago I worked for a travel agency in Bangkok when my boss called me in his office. "What do you know about Greek mythology?" he asked. Well, the basics, I assume. "Find out what you can about Poseidon and Cape Sounion, then."
It appeared we had a very rich Thai lady who had a dream. In it Poseidon told her to make a sacrifice in his temple. She went to many travel agencies who couldn't help her. (They were polite enough not to mention visiting a doctor rather than a travel agent.) So I dug into it. That was when the Internet was in its infancy. Little to be found, text only. No Google, no Yahoo. But I pulled it off, and we could satisfy her request. I think it was one of the strangest tours I ever worked on.
To relate this story to the question: this lady was Thai. Animism is normal in Thai culture. One's superstition is someone else religion. We had (and have) plenty of PM's who won't do anything unless their soothsayers approves. The lady wasn't into Greek mythology at all. She knew who Poseidon was. That's about it. But she took that dream very serious. I wouldn't say that counts as revival of an ancient religion, but these things happen.
Another reason why this question is so difficult to answer.