The needed specs depend on the modifications of the server and the amount of players it got.
I got the Eco Lage X4 and I am currently hosting two TeamSpeak 3 servers, 1 minecraft server and 2 Ragnarok Online servers, both with hard modifications requiring many database transactions etc. and the current server load could handle two more Ragnarok Online servers with the same requirements as the two that are running already.
All 4 could take up to 100 players lagless at the same time I guess.
100 doesn't sound like much, but actually, there aren't many servers that even reach the 80 player line, getting higher specs would be wasted money.