The amount of reputation required to increase Renown has been increased in the latest Dragonflight Beta Build.
There are four main Renown factions in Dragonflight. The maximum Renown is 30 on launch. To increase your Renown by 1, you must currently fill the bar to 2,500 reputation. You gain reputation by completing quests (main campaign and side quest included).
In the previous build, 1,000 reputation was required to increase your Renown.
Fortunately, Renown achievements are account-wide, so unlocking the rewards on one character will automatically unlock them for your alts.
This change was discovered by content creator MrGM.