stocking is really dependant on how you run your tank. if the tank depens soley on nitrifying bacteria aka (beneficial bacteria) to process waste, then I recommend you stick with the one betta. if you plant your tank very well(until you cant see the back wall of your tank... or the sides) and the plants are thriving, the tank will be able to take a lot more bioload.
since you got a kit, I would recommend looking into planted tanks, as these needs much less maintainance and has better stability than the non live plant counterpart
And the most amount of happy healthy fish I've ever seen in a 5.5? 30+ ...
but i don't recommend you to try more than an oto or two until you become a real professional at this, that guy knew what he was doing :)
Either way. get that tank cycled first before you get anything extra