In my recent game I got 3 leaders during a 20-turn war with Germany (they were #1, got wiped out
)
The first was used to build an Army
The second was used to build the FP in a captured german city (that's why I didn't build it with the first leader)
The third was used to build the Sistine Chapel.
Then I got a 4th one about 20 turns later in a minor skirmish with the Russians. It was used to make Adam Smith's Trading Co.
Now I am at war with England and got another one!! It is in Paris waiting to get a new wonder once I get the tech.
I would like to make note that my decision to build an Army in this case was an exception, I was fighting a war against a civ which was #1, had over twice as many cities and its territory was at least 3 times mine. As such there was a risk in my attack bogging itself down significantly.
