The maximum level a champion can reach in a normal game is 18.
In League of Legends, champions begin a standard Summoner’s Rift match at level 1 and gain experience from nearby minions, champion takedowns, and jungle monsters. Each level improves a champion’s statistics and provides ability points, allowing players to strengthen basic abilities and eventually upgrade their ultimate.
Level 18 is the normal-game ceiling because champions have only 17 level-ups available after starting at level 1. Once a champion reaches that cap, further experience does not produce another level, although gold, items, buffs, and other advantages can continue to increase their power.
This answer applies to the usual Summoner’s Rift rules. Other modes can use different starting levels, experience systems, or limits, so answers such as 30 may reflect confusion with a different game or with the player’s account level. A champion’s in-match level is also separate from a player’s account level.