Limited styles in the Monochrome plot theme

One solution would be to double the number of plot styles under this theme by making them gray. So you would have 8 black styles, followed by 8 gray styes.

Learning from the answers here, you could do this via

System`PlotThemeDump`resolvePlotTheme["Monochrome", "Plot"] = 
 ReplaceAll[
  System`PlotThemeDump`resolvePlotTheme["Monochrome", "Plot"],
  HoldPattern[Rule["DefaultDashing", dashing_List]] :> 
   Rule["DefaultDashing", Join[dashing, Thread[{Gray, dashing}]]]];

Now you can have up to 16 distinct styles,

f[x_] := x^4 + x
g[x_] := x^2
tSet = Table[f[x] + c*g[x], {c, -4, 8}];
Plot[Evaluate@tSet, {x, -2, 2}, PlotTheme -> "Monochrome", 
 PlotLegends -> "Expressions"]

PlotStyle -> (Dashing[#]& /@ Table[i, {i, .01, .09, .01}])

or whatever style you like:

{Dashing[{.02, .05, .01}],Dashing[.04], Dashing[{.01, .02, .01, .06}],...}

or of course AbsoluteDashing