Horses

Way can't I make more units with horses? When I played Civ II I could have all the all the units with horses.

In Civilization V, the horses are limited. When you see the horses at the map, with the mouse over them, there is a number (usually 2 to 6, the maximum of units you can train with this resource). The same happens with all estrategic resources, like iron, coal, alumminium, uranium...

What makes the game a little unique is that you don't buy units with resources like I think most games are like, you produce (or buy at a premium with gold) units, and then must maintain them over time with resources (through a trade network). So they aren't depleted, you just need to have them to keep the units at full force.

You can see the number of resources available by glancing at the icons at the top of the screen.