Question

by how many electors should the nomination paper of a president candidate be proposed and seconded?

Answer 1

The electoral college for the election of President of India consists of elected members of the Parliament and elected members of the legislative assemblies of all states. Only 2 Union territories (Delhi and Pondicherry) have legislative assemblies and the elected members there also form part of the electoral college to elect the President of India. Now, the candidature of each candidate running for Presidency should be backed by 100 people from this electoral college - 50 as proposers and 50 as seconders. This is just to ensure that candidates who are not considered as favourable at all to not even get the backing of 100 electors are not included in the race for President. 
I hope I managed to clear your doubt! :)