For the Langmuir Isotherm model, from the experiment you have:

C0 (the initial concentration of the substance in solution (mg/L)),

Ce (the final concentration of the substance in solution after adsorption (mg/L))

Then calculate qe (the equilibrium concentration on the adsorbent (mg g-1)) from the equation:

(C0-Ce)*V/W (V is the volume of solution in L, w is the mass of adsorbent in g)

Then Langmuir is the plot of Ce versus Ce/qe.

From here you get a slope within the linear equation of y= mx+b from Microsoft Excel or OriginPro.

Finally, replace the parameters of the Langmuir equation in the linear equation which gives you: 1/qmax = slope, 1/qmaxKL = Intercept

So, your qmax is 1/slope