It is possible to change the number of offers a user can make against a Make Offer item within the Auctioneer settings. Click here to see how to access your Auctioneer settings.
Once you are editing your auctioneer, go to the Main Settings tab and scroll down to the Bidding section. Here there is a field named Max Offers per Item. The default is 3, but this can be increased or decreased as required, up to a limit of 99 offers:
When any required changes have been made, click the green Save Auctioneer button at the top of the screen.