[ksa]

Предложу такой вариант...

const dt = [
    [1, 2],
    [1, 3],
    [2, 4],
    [2, 3],
]
let min = dt[0][0]
let max = dt[0][0]
dt.forEach(el => {
	min = Math.min(min, el[0])
	max = Math.max(max, el[1])
})
let t = 0
for (let i = min; i <= max; i++) {
	let c = 0
	dt.forEach(el => {
		if ((el[0] <= i) && (i < el[1])) ++c
	})
	alert(i + ' - ' + c)
	t = Math.max(t, c)
}
alert(t)