I want to know that as my mother plants of potato are virus free , what are the chances / possible ways thous get infected by virus in multiplication cycle/ tissue culture
Taking your assumption into consideration "my mother plants of potato are virus free", your plants will stay virus free as long as:
1) they are in an environment that is free of all known virus vectors which include (Aphids, Beetles, Thrips, Leafhoppes, Whiteflies, Nematodes and Fungi).
2) They are kept away from virus infected materials.