How many engines could an Avro Lancaster survive on?

Jack Currie managed to land one with two engines dead on one side at West Malling.

He fouled up his first approach and had to go around again. He managed to get enough height over the fence so that they were safe, "as long as there were no double deck buses on the road". The rate of climb was slow and it was at this point his Bomb aimer told him that they still had a 4000 pound bomb on board. After getting back out over the Channel he dropped the cookie and landed on his second attempt

If you lose any more engines the only way is downward.

Ray

No payload, low of fuel, all unnecessary weight jettisoned incl mg's.... it could maintain forward airspeed sufficient to keep it 'flying' on 2 engines... albeit losing altitude rapidly.

"... Meanwhile the enemy fighter closed the range and fired again. A second engine of Captain Swales aircraft was put out of action. Almost defenceless, he stayed over the target area issuing his aiming instructions until he was satisfied that the attack had achieved its purpose. Captain Swales did not, however, regard his mission as completed. His aircraft was damaged. Its speed had been so much reduced that it could only with difficulty be kept in the air...

The aircraft, by now over friendly territory, became more and more difficult to control; it was losing height steadily. Realising that the situation was desperate Captain Swales ordered his crew to bale out. Time was very short and it required all his exertions to keep the aircraft steady while each of his crew moved in turn to the escape hatch and parachuted to safety. Hardly had the last crew-member jumped when the aircraft plunged to earth. Captain Swales was found dead at the controls."

http://www.bomber-command.info/vcswales.htm

None were it already on the ground. I recall it needed two were it flying. It would, of course, need the all to take off.