Yes, you need to buy 8 LEDs for each place.
i.e., 8 x 2 for Under Dashboard alone.
plus another 8 x 2 for rear floor lighting.
LED Cost = 32 x 2 = 64/-
Then you need some wires, soldering kit, PCBs small size, wires stripper and insulation tape.
Wires - 20/- (paired wires 4 metres)
PCB - 10 x 4 = 40/- (Ensure you can place 8 LEDs with enough space)
The below image shows a medium size one. You would need half the size of this.
In each LED one lead will be small and the other longer.
Insert the first 4 LEDs in one row such that previous LED's short leg is connected to next LED's long leg. Then follow the same procedure and insert another 4 LEDs in another row.
If you have a soldering kit and know to do soldering, pls proceed with soldering them
Otherwise just twist the adjacent legs and make the connection.
I didnt do soldering as my kit didnt work.
You will have the first LED's long leg and the last LED's short leg free. That is reserved for power supply.
Now you got a 4 LED series circuit. Repeat the same for the second row as well.
Make sure both the series has long legs on same side.
Then connect the two row's long legs on one end and short legs on the other end.
If you are twisting the legs to make the connection, make sure you do proper insulation such that it doesnt interfere with other connections. Connect 1metre of paired wire to this whole block. Red wire on the long leg and black wire on the short leg. Enclose this whole block into a small white box. Each box cost around 5/- only.
Put a hole in the box to let the wires out. Connect the wires to your car positive(red) and negative(black) terminal which is available in you dashboard. Stick the box with a double sided tape. It would cost 10/-.
Repeat the whole procedure to make 4 blocks and stick 2 of them under dashboard (left side and right side) and the other 2 under each front seat.