[lammps-users] Number of molecules.

Dear all ,

i am new both to Lammps and to MD.I want to do some simulations which may for example CO2 or H2O. I read in the manual that i can define an initial volume from the data file with xlo,xhi,ylo,yhi,zlo,zhi which create a volume V.I want to use a NPT ensembe at first with T=300K and P=1atm for example.How can i know how many molecules can i put inside my box or i don’t care because i use an NPT and the volume will change? Are there Tables where i can find what is the density under specific T and P? How can i do the same if i use the NVE about the total number of molecules in order for all the above to have my CO2 in a vapour condition?