ENIAC used roughly 18,000 vacuum tubes

ENIAC, the Electronic Numerical Integrator and Computer, was built at the University of Pennsylvania and unveiled in February 1946 as the first general-purpose electronic digital computer. According to Penn Engineering’s own history of the machine, it used 18,000 vacuum tubes and weighed 30 tons, occupying a room about 30 by 50 feet. The tubes switch far faster than mechanical relays, which is why the machine was so quick: Penn notes that a ballistics calculation that took 12 hours on a hand calculator could be done in about 30 seconds. ENIAC also consumed a great deal of power. The University’s account is the first-party source for these specifications; the ENIAC patent (US 3,120,606) is the original engineering record of the design but does not state the tube count and weight as plainly.