How many objects is "too many" for in a single transaction to Google's DataStore (High Replication)?

I have following entity (non-relevant fields/methods are removed).

public class HitsStatsTotalDO
{
    @Id
    transient private Long targetId;

    public Key<HitsStatsTotalDO> createKey()
    {
        return new Key<HitsStatsTotalDO>(HitsStatsTotalDO.class, targetId);
    }
}

So... I'm trying to do batch get for 10 objects for which I construct keys using HitsStatsTotalDO.createKey(). I'm attempting to fetch them in transaction like this:

final List<Key<HitsStatsTotalDO>> keys = ....

// This is being called in transaction..
Map<Key<HitsStatsTotalDO>, HitsStatsTotalDO> result = DAOBase.ofy().get(keys);

which throws following exception:

java.lang.IllegalArgumentException: operating on too many entity groups in a single transaction.

Could you please elaborate how many is too many and how to fix it ? I couldn't find exact number in the documentation.

Thanks!