Birthday paradox.

birthday_paradox.js

// https://betterexplained.com/examples/birthday/birthday.html
function calc(people) {
    let ret = {
        days: 365,
        people: people
    }

    ret.combinations = ret.people * (ret.people - 1) / 2
    ret.chance = (ret.days - 1) / ret.days
    ret.expected = 1 - (ret.chance ** ret.combinations)
    ret.percent = ret.expected * 100

    return ret
}

let people = 100, percent, min_percent = 50.0
for (let p = 1; p <= people; p++) {
    percent = calc(p).percent
    if (percent >= min_percent) {
        people = p
        break
    }
}

//=> Person: 23, Percent: %50.05
console.log("Person: %s, Percent: %%%s", people, percent.toFixed(2))