How much current can I draw from a battery?
From the battery specification that you posted it says that the maximum continuous discharging current is 1000mA. Or 1A if you convert the units. So for safe use of the battery and safety to yourself you would not want to exceed this amount.

You were asking about using a boost converter to increase the battery voltage to 12V. A well designed boost converter would be able to achieve an efficiency of say 85%. This means that:

Power_Out = 0.85 * Power_In

Power_In = Vbat * 1A max = 3.7V * 1A = 3.7W max.

Power_Out = 0.85 * 3.7W = 3.14W max.

Current_Out = 3.14W max / 12V = 0.26A max.

This means that you must not place a load on the boost converter of more than 260mA in order to stay within the safe operational zone of the battery.