documentclass{article}
% RN. 19 November 2017
%=======================
usepackage[check-declarations]{expl3}
usepackage{xparse}
usepackage[T1]{fontenc}
%-----------------------
ExplSyntaxOn
int_new:N l_rn_i_int  %  local, for looping

tl_new:N fr_rn_buildVariable_tl
cs_new:Npn rn_buildVariable_fr:n #1
%  PARAMETER:  #1 the number of entries per row
%  RETURNS: fr_rn_buildVariable_tl
  {
group_begin:
  tl_gclear:N fr_rn_buildVariable_tl
  tl_gput_right:Nx fr_rn_buildVariable_tl {|}
  int_set:Nn l_rn_i_int {1}
  int_until_do:nNnn {l_rn_i_int} > {#1}
    {
      int_use:N l_rn_i_int\
      tl_gput_right:Nn ~fr_rn_buildVariable_tl {col.~int_use:N l_rn_i_int |}
      int_incr:N l_rn_i_int
    }
group_end:
  }  %  rn_buildVariable_fr:n

NewDocumentCommandmyTLVariable{O{5}}
%  PARAMETER:  #1 nr of items in tl_ variable
  {
group_begin:
  MWE\
  rn_buildVariable_fr:n {#1}
  fr_rn_buildVariable_tl
group_end:
  }  %  myTLVariable
ExplSyntaxOff
%-----------------------
begin{document}
  myTLVariable
  myTLVariable[8]
end{document}