XGBoost and Random Forest: ntrees vs. number of boosting rounds vs. n_estimators
As I understand it, iterations is equivalent to boosting rounds.

However, number of trees is not necessarily equivalent to the above, as xgboost has a parameter called num_parallel_tree which allows the user to create multiple trees per iteration (i.e. think of it as boosted random forest).

As an example, if the user set num_parallel_tree = 3 for 500 iterations, then number of trees = 1500 (=3*500) rather than 500.