Listbox populate with specific rows

I want to populate a vba listbox from a database only with specific rows. This is what I got.

Private Sub UserForm_Initialize() 

Hoja2.Activate
ListBox1.ColumnCount = 5
ListBox1.ColumnWidths = "70;90;90;90;70"


ListBox1.AddItem "FIRST NAME"
ListBox1.List(0, 1) = "LAST NAME" 
ListBox1.List(0, 2) = "LAST NAME 2"
ListBox1.List(0, 3) = "BORN DATE"
ListBox1.List(0, 4) = "AGE"


Dim seguimiento As Integer
Dim i As Integer

seguimiento = Application.WorksheetFunction.CountA(Range("b:b"))

For i = 1 To seguimiento
    If Cells(i, 20) = "" Then
    ListBox1.AddItem Cells(i, 3)
    Else
    End If
Next i

End Sub`

it's actually hard to understand listbox filling criteria from your question

assuming you want to fill listbox with cells:

on rows whose column "T" cell is not empty
scanning rows from row 2 down to last non empty row in column "B"
getting values form columns "C" trough "G"

try the following code:

Option Explicit

Private Sub UserForm_Initialize()
    Dim seguimiento As Long, i As Long
    Dim Data() As Variant '<--| use an array to store data to eventually fill ListBox list
    Dim cell As Range

    With Me.ListBox1
        .ColumnCount = 5
        .ColumnWidths = "70;90;90;90;70"
    End With

    i = 1
    With Hoja2 '<--| refer to your worksheet
        With .Range("T1:T" & .Cells(.Rows.Count, "B").End(xlUp).Row).SpecialCells(xlCellTypeConstants) '<--| refer to non blank cells in its column "T" (i.e. with column index 20) from row 1 to last non blank one in column "B"
            seguimiento = .Count '<--| count those non empty cells
            ReDim Data(1 To seguimiento + 1, 1 To Me.ListBox1.ColumnCount) '<--| redim data array rows accordingly (while setting columns to 5)

            'fill data array first row with "headers"
            Data(1, 1) = "FIRST NAME"
            Data(1, 2) = "LAST NAME"
            Data(1, 3) = "LAST NAME 2"
            Data(1, 4) = "BORN DATE"
            Data(1, 5) = "AGE"

            ' loop through referred non blank cells and fill subsequent data array rows from corresponding cells in columns "C" through "G"
            For Each cell In .Cells
                i = i + 1
                With cell
                    Data(i, 1) = .Offset(, -17) '<--| this refers to a cell in the same row as the current cell and in column "C", being offseted 17 columns back from current cell
                    Data(i, 2) = .Offset(, -16) '<--| this refers to a cell in the same row as the current cell and in column "D", being offseted 16 columns back from current cell
                    Data(i, 3) = .Offset(, -15) ' etc...
                    Data(i, 4) = .Offset(, -14) ' etc...
                    Data(i, 5) = .Offset(, -13) ' etc...
                End With
            Next cell
        End With
    End With

    ListBox1.List = Data '<---| finally, fill listbox list with data array
End Sub