Any number of columns may be inserted at the same time. While the insert() method only accepts one column, batch_mutate() allows you to update or insert multiple columns at once.
When inserting or updating multiple keys in one operation (using batch_mutate(), for example), it is important to note that operations in Cassandra are only atomic at the row level. That is, an operation on different rows at the same time will not be atomic.
Columns are only overwritten if the timestamp in the new version of the column is more recent than the existing column, so precise timestamps are necessary if column overwrites are frequent.