pressure in the air is about 0.95 atm. you have to inflate the balloons with air from your lungs. When you do this, the air exits your mouth at roughly your body temperature which is about 98F, and it has the same pressure as the surrounding air.

Each balloon requires about 2.5 L of air from your lungs.

After the air goes into each balloon, its volume starts at 2.5 L and its temperature is 98F, but its temperature eventually comes to equilibrium with the ambient air temperature resulting in a drop of about 40F, and its pressure approximately equals the ambient air pressure the whole time.

how much heat flows to it from the ambient air before it reaches the equilibrium temperature?

There are 101,325 Pa per atmosphere, 1000 L per cubic meter, and R ~ 8.31 K/ (mol K).