I believe it's only 1, because after it gets an "x" it no longer can be used for attribute material. The only thing I think attribute does is increase the health of the card, please correct me if I'm wrong, I wish there was a super detailed guide somewhere to understand those mechanics
2 years ago (+3)
You had a good question so i made a quick research for you maybe it helps.
There must be a intern multiplier wich will be increased with each heart at the top.
At 3 Hearts the multiplier is round about 1.4 so u can multiply atk/def/life with this 1.4 and can calculate how strong the pokemon will with 3 hearts.
The multiplier is not exactly 1.4 in this sheet maybe because of rounding stuff only the owner knows the truth.