You'll find both, and both are grammatical. The intertubes says that "up to" is by far the less popular. A brief look leads me to believe that the "number to" form is used when there's no expectation for the total: I just saw another hybrid car.
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The "number up to form" seems to be used when the number fills a lack. TI just saw another hybrid car. That brings the total I've see today to five.