This article should give an idea of the likelihood of pregnancy based on the number of sperm available. If you look at the graph of the Probability of Conception vs. the Sperm Concentration, it shows that at more than 10 million sperm/mL (or around 30-40 million total sperm, estimating average semen volume to be 3-4 mL), the probability of conception per month quickly reaches a plateau, while at under this concentration there is a roughly linear drop in the probability of conception to zero at zero sperm. Another study shows a high variability in the number of sperm present in pre-ejaculate. Of the 28 men tested, only 12 produced any detectable sperm in their pre-ejaculate at all. Of these 12, the sample with the highest number of sperm contained 35 million (subject 17). If this man consistently produced this many sperm in his pre-ejaculate, I would estimate the probability of him causing conception per month as 35 million divided by 35 million (from above) multiplied by ten percent, i.e., 10%/month.
In other words, it is possible, according to these studies, for pre-ejaculate to cause pregnancy. However, this evidence also suggests that some men, if their lack of sperm in pre-ejaculate is consistent, may be unable to cause pregnancy in this way.