How many roundtrips are made to a MongoDB server when using transactions?

I wonder how many roundtrips that are made to the server when using transactions MongoDB? For example if the Java driver is used like this:

ClientSession clientSession = client.startSession();
TransactionOptions txnOptions = TransactionOptions.builder()
        .readPreference(ReadPreference.primary())
        .readConcern(ReadConcern.LOCAL)
        .writeConcern(WriteConcern.MAJORITY)
        .build();

TransactionBody txnBody = new TransactionBody<String>() {
    public String execute() {
        MongoCollection<Document> coll1 = client.getDatabase("mydb1").getCollection("foo");
        MongoCollection<Document> coll2 = client.getDatabase("mydb2").getCollection("bar");

        coll1.insertOne(clientSession, new Document("abc", 1));
        coll2.insertOne(clientSession, new Document("xyz", 999));
        return "Inserted into collections in different databases";
    }
};
try {
    clientSession.withTransaction(txnBody, txnOptions);
} catch (RuntimeException e) {
    // some error handling
} finally {
    clientSession.close();
}

In this case two documents are stored in a transaction:

coll1.insertOne(clientSession, new Document("abc", 1));
coll2.insertOne(clientSession, new Document("xyz", 999));

Are the "insert operations" stacked up and sent to the server in one roundtrip or are two calls (or more?) actually made to the server?

Each insert is sent separately. You can use use bulk writes to batch write operations together.

The commit at the end is a separate operation also.