Multiple infrared LEDs in parallel or series?
Series is always the best, you waste less power on the resistors, your BJT needs to handle less current.
Also the forward voltage of LEDs may vary quite a bit, which will make some LED draw more current than others and generate different brightness. This will not happen when in series as they will all share the same current. Brightness is directly correlated to the current, not the voltage.
If you can (have a higher voltage rail than 5V if it's battery-powered), put everything in serie.
Higher voltage is always easier to handle than higher current (within reason).