One for life, 5-7 at the size of quarters but plan on a bigger tank within 6 months, If you start with a 55g you could strecth that to a year but will need at least a 75g for 5 adults, preferably larger, a 125g can keep 7 for life easily.
By the way 50cal, welcome to MFK,
the rifle I most enjoyed firing was the semi-auto Barrett m82A1 .50 BMG, 10 shot box magazine fed, 10 hits in a 10" circle at 1,000 meters on a calm day with a 20x scope, it is an expensive toy good only for targets, (or maybe armored cars) that costs $7500 US new in box, and $3-$4 US a round to shoot. Handloaded match grade 620 grain hand turned solid copper slugs loaded to a muzzle velocity of 2,99ft per second cost $15 US each and the unloaded rifle weighs just over 28 pounds.