To elaborate on my suggestion here's a more thought out plan.
Iron blocks would fall from the ceiling once the player count would be low enough. There would be a specific amount of players based on how many people are playing. Here's a rough chart of what it would be in my opinion. The ratios are players playing to amount of players until iron blocks drop. For example, if 20 people are playing that game, that means that iron blocks would start falling when there are 5 players left.
20 = 5 players
19 = 5 players
18 = 5 players
17 = 5 players
16 = 4 players
15 = 4 players
14 = 4 players
13 = 4 players
12 = 3 players
11 = 3 players
10 = 3 players
9 = 3 players
8 = 3 players
7 = 3 players
6 = 2 players
5 = No iron blocks
