Your choice of word must agree with "volunteers", so it has to be "are".

If you want one volunteer, you can say "Is there a volunteer?" (or the more idiomatic, "Can I have a volunteer?").

There is no simple construct that implies "I will accept one but would like more". You would have to say that explicitly. e.g. "Can I have a volunteer? Preferably more than one..."

Another way to do this would be, "Are there one or more volunteers?" But in this case, you really could argue about whether "is" or "are" is correct! (Does it agree with "one"? Or with "more" and "volunteers"?)