Unfortunately, Zoho has set the custom lookup fields to be 5.

However, there are several modifications to this like multi-select look-ups and automating the associated module building.

There is also the option to go into the "Unused" fields when building out a module, you can check if there are look-ups not being used. By deleting one of those you can get 1 lookup back. Please note that by doing so (deleting a field in the unused section) you will loose the data within it forever.  

There is still the limit of 5 lookup fields, but it depends on what you want/need to do as to whether this limit will impact you or not.