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A model of this airplane was used to carry a space shuttle. In this picture below the airplane had a few extra engines added since then.

How is the number of engines decided on an airplane? Why not use bigger engines?

enter image description here I think this picture maybe fake? ANTONOV 925

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When are more engines better than not on a plane? When one or more of them has quit working.

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It's a fake yes. A simple search for model number 925 shows no such plane existed, nor it could with the lower wing covered like that.

Fewer engines are better. It reduces complexity, and improves statistical reliability (Lusser's law).

Two is minimum for jetliners, because still an improved reliability does not mean a single engine won't fail, so it's better to have another to rely on.

The reason for 8 (B-52), 6, 4, 3 (a bit more complicated as it can be a workaround for pre-ETOPS era) is the available technology at the time of designing the plane.

That is why one of the lightest quads was an old giant plane (engine technology hadn't matured yet past 160 hp).

A 777 could not have two engines until the technology grew enough to make the most powerful turbofan.

Related: Are two twin-jet flights more efficient than one quad-jet flight? (Keep in mind that there are costs other than fuel that increase with the number of engines; [systems] complexity for one and the associated maintenance.)

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  • $\begingroup$ Very nice fact filled answer. $\endgroup$
    – Muze
    Jul 7, 2019 at 19:16
  • $\begingroup$ Engines that travel long distances like the B-52 under a heavy load need more engines not because 2 could not do the trick but mechanical failure is less with the work load over more engines and some extra power may have been needed for evasive flying? $\endgroup$
    – Muze
    Jul 7, 2019 at 19:29
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    $\begingroup$ @Muze: A 777 can climb with one engine, such things are accounted for in the design. And can fly for hours (ETOPS) on one engine. (Technological advances.) $\endgroup$
    – user14897
    Jul 7, 2019 at 19:41
  • $\begingroup$ Yes. Today not back then. Tks. $\endgroup$
    – Muze
    Jul 7, 2019 at 19:55
  • $\begingroup$ Your reference to Lusser's law reminded my of a young field reporter on TV talking about a local incident a few years ago. With furrowed brow and sincere voice he dropped the statistical bombshell that twin engine airplanes are twice as likely to experience engine failure! (Neglecting, of course, to mention the simple fact that a twin still has one left when this happens.) $\endgroup$
    – Michael Hall
    Jul 8, 2019 at 15:43

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