I am going to buy a cycle. In this regard I got my framesize 20". Now I would like to know is it for 26er or 29er ?? Is frame size will be different for 29er and 26er ??

I thought 20" was 20" for whatever wheel size
Has more to do with seat to pedal distance, pedal length & standover height.
Road bikes are a bit different, the 20" probably don't apply.

Check "standover" clearance on a mtb. If you have to "Flintstone brake" Good to have an inch or 2

FYI, I got a 29er, I don't do the highly technical single track tails,
The 29er is more comfortable ride, bigger tires go over bumps better & have over 1000 mile riding paved roads with it.

If buying from a bike shop, they should fit you
If not, have a bike shop fit you, then buy wherever.
If a bike shop treats you well, you'll give them your business & go there for parts & service

Frame size is independent of wheel size, and you should consult the manufacturer spec's. In most cases, a 20" frame is for people over 6' tall.
Let's say that you were to put 29" wheels on a frame intended for 26" wheels, that would change the framing size enough to cause pain from the top bar were you to hop off suddenly without thinking...

Go take test rides in a bike shop. & see how the different types fit.. unsaid is a 20" what.. BMX Wheel?
have you heard of the Metric system?