REST api request and loop by offset until no further records found

First, check the request metadata to see if it tells you how many records there are. 

E.g. 

let

   webData = Web.Contents("https://..."),

   webMetadata = Value.Metadata(webData)

in

   webMetadata

For doing a while loop in power you can instead use recursive functions like:

let
   my_func = (startIndex) =>
      let
          webRequest = Web.Contents(...),
         ....,
         results = ...,
         numRecords = ....,
         if numRecords = page_size then
             results & my_func(startIndex + page_size)
         else
              results
in
   my_func(0)

 Note the @ used for recursion