How to get all pages into a single result

I have a following API call

this.httpCall.getAllItems().subscribe(results => {
  for (const each of results) {
    .....
  }
 ....
});

The getAllItems() is a basic HTTP API call which returns a result of 10 records by default. The total count comes in header X-Total-Count which for example is 65

getAllItems(): Observable<any> {
  const url = `URL/items?count=true`;
  return this.http.get(url, { observe: 'response' }));
}

The total count comes in results.headers.get('X-Total-Count');.

To get the next page I need to send offset in API. So, to get all 65 records, I would do

URL/items?limit=10&offset=0
URL/items?limit=10&offset=10
URL/items?limit=10&offset=20
URL/items?limit=10&offset=30
URL/items?limit=10&offset=40
URL/items?limit=10&offset=50
URL/items?limit=10&offset=60

At

URL/items?limit=10&offset=70 

There will be 0 return.

How do I implement this above in this.httpCall.getAllItems().subscribe(results => so that I can join all the results in one and then do the for loop continuation?

I read that perhaps switchMap can help here, but I need some help on how to implement it in above logic.

First of all you will need a call to get items with offset as a parameter.

getItems(offset : number): Observable<any> {
  const url = `URL/items`; 
  return this.http.get(url, { 'limit': 10, 'offset': offset }));
}

you will also need a call to get the number of total records

getNumberOfItems(): Observable<any> {
  const url = `URL/items?count=true`;
  return this.http.get(url, { observe: 'response' }));
}

then subscribe to it and call the getItems function with offset as parameter in a for loop

this.httpCall.getNumberOfItems().subscribe(result: any => {
    let allItems = []
    const totalCount = result.headers.get('X-Total-Count');
    const iterations = Math.ceil(totalCount/10) 
    for (let i = 0; i < iterations; i++) {
        this.httpCall.getItems(i*10).subscribe(listOfItems: any => {
           allItems.concat(listOfItems) 
        })
    }
});

Find here what ceil does
and here what concat does