A question about L7805 voltage regulator

what is the maximum input voltage i can give to L7805 voltage regulator ?

It has a data sheet. You really have to go there because different packages and versions may have different specs.

oh.....what is a datasheet.....sorry i dont know that....as I am a pure NOOB...just started with electronics....can u give me the link..or something for datasheet?

Google "7805 datasheet"?

A datasheet is the documentation for a part.

Voltage you can apply varies with the current load, and with the ratings of the part.
For example, a part may handle 20V in with 5V out if the current draw is low.
The part will dissipate the excess voltage as Power = (Vin - Vout) x current draw.

As current draw increases, power dissipated rises, and at some point the part will overheat and shutdown (or fail).
Heat sinks can be added to help with cooling. Could be a clipped on part, could be a nut & bolt on part, could be attached to the side of a chassis, could use copper in the PCB it is mounted on.

There are several models of L7805 in different kinds of packages. Which do you have?
https://www.digikey.com/products/en/integrated-circuits-ics/pmic-voltage-regulators-linear/699?k=l7805

Here is a link to the L78xx voltage regulator datasheet.

Read it.

There is a table on page 4 that will help you:

L7805 Max Input Voltage.png

Note that an "absolute maximum" rating is not a value you'd ever use. If the absolute maximum input voltage is 35V, use it upto 28V or so.

Power dissipation will be a very important issue if you drop lots of volts across a linear regulator. Normally a buck-converter would be used for a large drop in voltage, possibly before a linear regulator (if you want a low
noise output).