Roughly thirty names. Everything else in the library is either a variation on one of these or something you will look up once and forget until you need it again.
The imports that cover most work
from sklearn.model_selection import (train_test_split, StratifiedKFold,
TimeSeriesSplit, GroupKFold,
cross_validate, RandomizedSearchCV,
learning_curve)
from sklearn.pipeline import Pipeline
from sklearn.compose import ColumnTransformer, make_column_selector
from sklearn.impute import SimpleImputer
from sklearn.preprocessing import StandardScaler, OneHotEncoder
from sklearn.feature_extraction.text import TfidfVectorizer
from sklearn.linear_model import LogisticRegression, Ridge
from sklearn.ensemble import (RandomForestClassifier,
HistGradientBoostingClassifier)
from sklearn.svm import LinearSVC
from sklearn.dummy import DummyClassifier
from sklearn.metrics import (classification_report, confusion_matrix,
average_precision_score, roc_auc_score,
ConfusionMatrixDisplay)
from sklearn.inspection import permutation_importance
from sklearn.calibration import CalibratedClassifierCV