What is the best way to infuse garlic flavor in the bland brown rice?
Garlic flavors are soluble in oil, simply chopping up garlic and putting it in with the rice will not distribute the flavor well. I would recommend you chop the garlic finely and then infuse into a tablespoon of oil or butter by putting the pan on very low heat for a few minutes. Then add the rice and stir to coat the grains with the infused oil before adding water and cooking as normal. 

How much garlic to use depends on how strong your garlic is and how garlic-y you want it to taste. 5 tbsp of rice isn't much, for a medium garlic flavor using average strength garlic a whole small clove or half a large is probably enough. With the garlic I grow I'd add a few slices from a clove at most. 

Another option would be to cook the rice and then stir in the garlic infused oil afterwards, The benefits to this option are that you can drain the rice after cooking without all the flavor going down the drain.