When veneers are placed on front 75% tooth, can it cover the remaining 25% of back side or it will just cover 25% front side?

Does not matter: Veneer is design such that it would give the best retention with minimal tooth removal and have minimal exposure area of direct occlusal force contact. The margin of veneer on the back side should avoid the occlusal contact if possible. Don't worry about how many percent it covered but emphasize on how long it should last. With a good design and good bonding system, the result is long lasting.

Get informed: The very thin veneers you are speaking of will cover 100% of the front side and none of the back. A lets say "normal" veneer prep these days is a prep that covers 100% of your front and 75 % of the back and in between your teeth. The wrap around effect makes them stronger and look better but takes more of your tooth. The thin veneers youre speaking of require little to no prep . Get informed (dds).

Either way: Veneers can be very versatile. It depends on the design the cosmetic dentist chooses. Sometimes we use veneers to cover the worn and damaged part of the tooth as well as to improve the cosmetics of the patients smile. What confuses me regarding your question is that veneers are placed on the front 75% of the tooth? Usually veneers cover the entire front surface of the tooth and back depends.

Veneer vs crown: Veneer are used to do minimal invasive shaving and they usually cover the front, sometime they cover the contact crown are used to wrap the hole tooth.