f[Range[8]]
Flatten[Table[f[%[[p]]], {p, Length[%]}], 1]
Flatten[Table[f[%[[p]]], {p, Length[%]}], 1]

{{1, 2, {3, 4}, 5, 6, 7, 8},
 {1, 2, 3, {4, 5}, 6, 7, 8},
 {1, 2, 3, 4, {5, 6}, 7, 8}}

{{1, 2, {{3, 4}, 5}, 6, 7, 8},
 {1, 2, {3, 4}, {5, 6}, 7, 8},
 {1, 2, {3, {4, 5}}, 6, 7, 8},
 {1, 2, 3, {{4, 5}, 6}, 7, 8},
 {1, 2, {3, 4}, {5, 6}, 7, 8},
 {1, 2, 3, {4, {5, 6}}, 7, 8}}

{{1, 2, {{{3, 4}, 5}, 6}, 7, 8},
 {1, 2, {{3, 4}, {5, 6}}, 7, 8},
 {1, 2, {{3, {4, 5}}, 6}, 7, 8},
 {1, 2, {3, {{4, 5}, 6}}, 7, 8},
 {1, 2, {{3, 4}, {5, 6}}, 7, 8},
 {1, 2, {3, {4, {5, 6}}}, 7, 8}}

NestWhileList[
 Flatten[Table[f[# &[[p]]], {p, Length[# &]}], 1],
 f[Range[8]], Length[#] & < 3!]

{{{1, 2, {3, 4}, 5, 6, 7, 8},
  {1, 2, 3, {4, 5}, 6, 7, 8},
  {1, 2, 3, 4, {5, 6}, 7, 8}}}