如何变为这种形式呢

n = 3;
Plot[ Table[Sin[x + a], {a, Range[2 n]/5 - .6}] // Evaluate, {x, 0, 2*Pi},
PlotStyle -> (Hue[#/10 + .3, 1, .8] & /@ Range[10]),
PlotLegends ->
Placed[LineLegend[Table["Leg" <> ToString[i], {i, 1, 2 n}],
LegendMargins -> 0,
LegendLayout -> (Row[{Grid[#[[1 ;; n]], Spacings -> {.5, .02}],
Grid[#[[n + 1 ;; 2 n]], Spacings -> {.5, .02}]},
" "] &)], {0.7, 0.8}] ]

其实还有个更简单的方法：
mathematica.stackexchange.com/q/267080/1871
LineLegend[63, Range[5], LegendLayout -> {"Column", 2}]