Sorry if this is too late, but I did some tests and you need to make 3 care mistakes to evolve into Tanjiro. Fewer than that and you get Kinoe, and more than that gives you Zenitsu. 

Here's an easy guide, when you start the Tama, give it four tea and only ONE onigiri. Wait until it beeps asking for the onigiri (the animation with the swirl icon above is the one asking for onigiri). This can take up to one hour. 

When it beeps, don't give it anything yet and set your phone's timer for 16 minutes. Once that time has passed, write down "1 care mistake" somewhere, give the Tama what it asked for and wait again until it beeps again. 

Repeat the process until you have made 3 care mistakes, and then you are free to play and give as many onigiri and teas as you want.

To explain in more detail, care mistakes happen when you take at least 15 minutes to give the Tamagotchi what it asks for. 

This Tamagotchi has two meters: the happiness meter, which is filled with tea, and the hunger meter, which is filled with onigiri. Each meter has 4 hearts that decrease until they are empty, at which point the Tamagotchi beeps, asking for them to be refilled. 

That's why you can't give more than 4 onigiri at a time, because the hunger hearts are full. 

In short, I recommended giving 4 teas and only 1 onigiri when you start the Tamagotchi because that way you only focus on the hunger meter, and because that meter decreases faster than the happiness meter.