This is for what I gather you are wanting to do
It looks like your wanting to show an "x" amount of textboxes for the value
selected.
Set the textboxes visible property to "No"

Try this in the combos after update event
If any orther code exist, you might want to delete the "exit sub" from each
if statement.

Private Sub myCombo_AfterUpdate()
If me.mycombo=1 then
   me.unit1.visible=true
   me.unit2.visible=false
   me.unit3.visible=false
   me.unit4.visible=false
   me.unit5.visible=false
   exit sub
End if
If me.mycombo=2 then
   me.unit1.visible=true
   me.unit2.visible=true
   me.unit3.visible=false
   me.unit4.visible=false
   me.unit5.visible=false
   exit sub
End if
If me.mycombo=3 then
   me.unit1.visible=true
   me.unit2.visible=true
   me.unit3.visible=true
   me.unit4.visible=false
   me.unit5.visible=false
   exit sub
End if
If me.mycombo=4 then
   me.unit1.visible=true
   me.unit2.visible=true
   me.unit3.visible=true
   me.unit4.visible=true
   me.unit5.visible=false
   exit sub
End if
If me.mycombo=5 then
   me.unit1.visible=true
   me.unit2.visible=true
   me.unit3.visible=true
   me.unit4.visible=true
   me.unit5.visible=true
   exit sub
End if
End Sub

This will only work on the current record

You may be able to do this without VBA if you use Conditional Formatting.
The problem is that the textbox would still "be there" if the user tabbed
into it or clicked in the correct location of the form. To do this using
Conditional Formatting, set the back color and fore color of the textbox to
match the color of the form. However, as you can see, this could create
confusion and problems for the user.

Using VBA, you would use the AfterUpdate event of the combo box and the
Current event of the form. The AfterUpdate event will catch when a user
changes the value in the combobox and the Current event will get the value
in the combobox as you move from record to record.

Example code:
Select Case Me.cboMyCombobox
   Case 1
       'Hide boxes as needed here
   Case 2
       'Hide boxes as needed here
   Case 3
       Me.txtUnit1.Visible = True
       Me.txtUnit2.Visible = True
       Me.txtUnit3.Visible = True
       Me.txtUnit4.Visible = False
       Me.txtUnit5.Visible = False
   Case 4
       'Hide boxes as needed here
   Case 5
       'Hide boxes as needed here
   Case Else
       MsgBox "Bad Value"
End Select

You really have to use VBA to set the text box's Visible
property.  Since you have nice uniform names for them, the
code in the combo box's AfterUpdate event procedure can be
very simple:

Dim K As Interger
For k = 1 To 5
    Me("Unit" & k).Visible = (k <= Me.combobox)
Next k

You're getting several good answers to the question - but I'd like to
"unask" the question. I suspect that the need to do this implies that
you have an incorrect table structure!

If textboxes Unit1 to Unit5 are bound to table fields Unit1 to Unit5,
then you have embedded a one-to-many relationship (one to five to be
exact) *in each record*. This is *bad design*! If you have a one to
many relationship, consider modeling it as a one to many relationship,
with two tables; use a Subform with five (or two, or eleven) rows for
the units rather than textboxes.

                 John W. Vinson[MVP]