Should i replace both or only one head gasket?

Replace both.
Yes, you can just do one head gasket, although I wouldn't recommend it.
If the head gasket has failed, it's usually because the engine has overheated. If you know that the engine has overheated, it would be best to do both heads.
When an engine overheats, the head warps a little. It doesn't take much to change the internal pressures on the gasket, and when the gasket area between the cylinders starts to lighten up a little, the gasket fails.
To do the job properly, take the head(s) to a qualified automotive machine shop and have the head resurfaced. Even a slightly warped head will be unable to keep a head gasket.
After a head has been resurfaced, it's relatively easy to put it back together.
NOTE!!
The machine shop will tell you the proper torque and torque sequence. Usually start from the center and work your way out evenly, going from side to side, and top to bottom, but get specific information for YOUR engine from the machine shop.
NEVER replace a head gasket without having the head resurfaced.

This details will help.Thanks.

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