The Army Multiplier is a setting that game creators can configure. It defaults to 10, which means that the first 10 armies each player purchases each turn cost 1 gold each. The next 10 cost 2 gold each, the next 10 cost 3 gold each, etc.
From the wiki, where 10 is the example.
{I do agree thru, Army Multiplier is not a intuitive name. However, I think almost all commerce games use the standard 10 as the setting}
29 gold, then means.
10 armies -> 10 g
10 +10 armies -> 10 g + 20 g = 30g
Hence with, 29 gold you can deploy 19 armies for 28 g. And you have 1g left