Were there 27 more Buddhas before Siddhartha Gautam Buddha? Were there 27 more Buddhas before Siddhartha Gautam Buddha? If so, are there any evidences of it. How about the finding of Buddhist Stupas and Idols in Harappa civilization?Is it linked with this fact?
Were there 27 more Buddhas before Siddhartha Gautam Buddha?

Yes, the Buddha speaks of them in his suttas, many of them from different times/planets throughout the universe(s).

If so, are there any evidences of it.

Other than the suttas themselves, yes in various other religious traditions.

How about the finding of Buddhist Stupas and Idols in Harappa civilization? Is it linked with this fact?

Yes it could be but rather than spending our lifetime endlessly trying to settle historical matters and learn other histories, let us be grateful for what the Gautama Buddha has provided us: a complete, methodical practice towards Enlightenment. We will need to devote whatever spare time aside from job and other duties to achieve what he achieved and the Buddhas after him (Padmasambhava). The other Buddhisms that stemmed from Hinayana Buddhism suggest there is a greater journey towards Complete Enlightenment and complete power, even after initial srotipanna and escaping the force of rebirth.
No need to excavate skeletons--it was hard enough for the Buddha to leave behind what he has and those after him.