How to create option or combo box within cell?

I don't think there's a misunderstanding, you should see all 10 in the drop down, as well as 11 and 12 and so on if you follow the instructions below:

Select the range where you want the drop down list to appear, for example E2:E20:

With that range highlighted, go to the Ribbon and choose > Data >>>Data Validation>>>List>>>

In the formula bar paste in:

=OFFSET(Ref!$A$1,1,0,COUNTA(Ref!$A:$A)-1,1)

Click OK

Now choose range I2:I20, make sure it is highlighted...

With that range highlighted, go to the Ribbon and choose > Data >>>Data Validation>>>List>>>

In the formula bar paste in:

=OFFSET(Agent!$B$1,1,0,COUNTA(Agent!$B:$B)-1,1)

Let me know if this is what you need...