The group planted 4 types of herbs: basil, thyme, rosemary, and oregano. Since they planted the same number of each type and had a total of 52 plants, we can divide the total number of plants by the number of types of herbs.
So, we calculate:
\[ \text{Number of plants of each herb} = \frac{52}{4} = 13 \]
This means that there are 13 rosemary plants.
Therefore, the answer is 13 rosemary plants.