How rare is it for a virgin to get pregnant who has never had any penile vaginal intercourse ?

Almost impossible: If semen is deposited in the vagina then pregnancy can occur. The most efficient way to get pregnant is through penile vaginal intercourse in which semen is directed deposited in the vagina. There are 4 million sperm in preejaculatory fluid (precum). It only takes one to get pregnant. Use condoms. Be smart and avoid these scares. Consider starting birth control to avoid an unplanned pregnancy.

not likely: In the recorded history of human kind this probably only happened once. There must be sperm introduced via the vagina that reaches the cervix for conception to occur. It is possible for conception to occur if someone places semen into the vagina with the use of a syringe or some other device and technically speaking no penis ever entered the vagina.