I am using fix Langevin as written in the Lammps documentation.
fix ID group-ID langevin Tstart Tstop damp seed keyword values
fix 1 all langevin 100.0 100. 1000 4827987

damp =1000, right.

Q1. My question is very simple what is the unit of this damp factor?
is it 1000fs or 1000ps ?

Q2. What is the relationship between damp and relaxation time?

Q3. Is relaxation time = damp*timestep?
if relaxation time is 50ps and timestep=10fs then Is it true that damp= 5000 ?

Q4. Suppose any paper mentions the Langevin friction coefficient is 0.01 ps^-1. then what is the value of damp?

Is it like this friction coefficient = mass/damp , let say mass = 1unit
so damp = 1/friction coefficient, i.e. , damp = 1/0.01 or damp = 100 ps ??

Q5 I want to know what is the default unit of damp factor in fix langevin.
all the questions, I assume in only real units.

The damp parameter is specified in time units and determines how rapidly the temperature is relaxed. For example, a value of 100.0 means to relax the temperature in a timespan of (roughly) 100 time units ( or fs or ps - see the units command). The damp factor can be thought of as inversely related to the viscosity of the solvent. I.e. a small relaxation time implies a high-viscosity solvent and vice versa. See the discussion about and viscosity in the documentation for the fix viscous command for more details.

That is what creates my confusion. damp = 100 time unit, so " in real unit " time unit is fs. It means that damp =100fs. Right ?

I read one paper where they were mention:- simulations were performed at 300K for 5μs, with a time step of 10 fs. A Langevin thermostat was used, with a relaxation time of 50ps.

They wrote their langevin thermostat like this
timestep 10
fix fxnve all nve
fix fxlange all langevin 300 300 5000.0 32784

here damp = 5000 that means damp = 5000fs or damp = 5ps but
they mention relaxation time = 50ps which implies relaxation time has some connection with the damp factor and time step.
i.e. relaxation time = damp*timestep
now if we check,
relaxation time = 50ps and time step = 10fs then damp factor = 5000.