As long as a master page is assigned to a document page you won’t be able to delete it. To start fresh, I would go to the Assign Page Range part of the Page Layout dialog and in the bottom part of it enter Page 1 to Page x (where x is your last page) and select “Standard” in the Master Page popup menu to the right. Then click Apply.
After this, the text box above should read P. 1–x: Standard, Newly added: Standard. Now you can select the unwanted master pages in the Master Page dialog and delete them.
First, go to Page Layout dialog and change the size of the of the header or footer area where your page numbers should go (say 0.5 inches). Don’t forget to click Apply.
Then double click on the header or footer area that you just created (it’s invisble, but you probably know where it should be). The main text will turn into a grey rectangle because you are now editing the master page.
Right click in the header or footer text frame and select “Insert page number” – a #-Placeholder will appear. Format the placeholder as needed (centered justification, font, font size, etc.). Click in the grey rectangle to get back to the main text. You should have numbered pages now, starting at 1.
Go to the Page Layout dialog again and select Page Numbering on the left side. Change the field “First page is number:” to -6. Keep the “Headers and footers start at:” 1. Click Apply. Now the first 7 pages (numbered -6, -5, -4, -3, -2, -1, 0) will not display headers and footers nor any page numbers therein, and the 8th page will start with page number 1.
For this you need a master page without page numbers as described earlier in this thread. Then go to the Assign Page Range dialog and in the bottom of it change Page 131 to 131 and select the “no page number” master page to the right. Then click Apply. Before you do this you might want to ckeck if the original page 131 has a different number now because you changed the page count previously.