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