it may be worth checking the current when your project is doing whatever it does
you mention a display device - can you measure the current while the display is changing?
battery capacity is amps * hours, but you should always allow some slack as not all batteries are equal, and the voltage may drop towards the end of its life
two batteries in series will give double the voltage - probably not what you want unless added together they give 5 volts
two batteries in parallel gives problems as they may not discharge evenly - and end up fighting
better to go for one battery that meets your needs
the battery in your link - hmm hard to see how it could be used!
maximum continuous current is just 2 mA
and it's only 3 volts!
3 or four of these
in series might work
why not just use 3 AA batteries? - simple, cheap