Used to be an ATW one that made 50 stops and an XC that made 45, both before COVID though and the various service changes that produced.
For most stops abroad, I think there are some in India with 70 or more.
I remember that old ATW service before covid. It was an evening Crewe to Cardiff Central via the Heart Of Wales line service and it took just over six hours to complete the entire journey. However dozens of the stations were request stops so in reality it probably rarely called everywhere. It served all 50 of these stations:
01 Crewe
02 Nantwich
03 Wrenbury
04 Whitchurch (Shropshire)
05 Prees
06 Wem
07 Yorton
08 Shrewsbury
09 Church Streeton
10 Craven Arms
11 Broome
12 Hopton Heath
13 Bucknell
14 Knighton
15 Knucklas
16 Llangynllo
17 Llanbister Road
18 Dolau
19 Pen-y-Bont
20 Llandrindod
21 Builth Road
22 Climeri
23 Garth (Powys)
24 Llangammarch
25 Llanwrtyd
26 Sugar Loaf
27 Cynghordy
28 Llandovery
29 Llanwrda
30 Llangadog
31 Llandeilo
32 Ffairfach
33 Llandybie
34 Ammanford
35 Pantyffynnon
36 Pontarddulais
37 Llangennech
38 Bynea
39 Llanelli
40 Gowerton
41 Swansea
42 Llansamlet
43 Skewen
44 Neath
45 Briton Ferry
46 Baglan
47 Port Talbot Parkway
48 Pyle
49 Bridgend
50 Cardiff Central
Pencoed / Llanharen / Pontyclun / Ninian Park are the only 4 stations that were skipped out. If it called at these then it would have been 54 stations.
There was also that old early morning SWT service from London Waterloo to Weymouth too. It also got scrapped just a few years before covid and took just over four hours to do the journey. This had zero request stops so the train actually stopped everywhere. It served all 48 of these stations:
01 London Waterloo
02 Clapham Junction
03 Wimbledon
04 Surbiton
05 Esher
06 Hersham
07 Walton On Thames
08 Weybridge
09 Byfleet & New Haw
10 West Byfleet
11 Woking
12 Brookwood
13 Farnborough Main
14 Fleet
15 Winchfield
16 Hook
17 Basingstoke
18 Micheldever
19 Winchester
20 Shawford
21 Eastleigh
22 Southampton Airport Parkway
23 Swaythling
24 St Denys
25 Southampton Central
26 Millbrook
27 Redbridge
28 Totton
29 Ashurst New Forest
30 Beaulieu Road
31 Brockenhurst
32 Sway
33 New Milton
34 Hinton Admiral
35 Cristchurch
36 Pokesdown
37 Bournemouth
38 Branksome
39 Parkstone
40 Poole
41 Hamworthy
42 Holton Heath
43 Wareham
44 Wool
45 Moreton
46 Dorchester South
47 Upwey
48 Weymouth
Vauxhall / Queenstown Road / Earlsfield / Raynes Park / New Malden / Berrylands were the only 6 stations it skipped out. If it called at these then it would have been 54 stations.
I'm guessing you're not counting trams? I'm not sure if they're records, either for trams alone or between trams and trains, but Line 67 in Melbourne has 57 stops (I think; I'm also not sure if that one has the Melbourne record) and Line 5 in Mannheim/Heidelberg has 75.
If we include trams then Paris Tram Line T1 will soon easily take the winning spot with exactly 80 stops once both the Eastern extension and the Western extension open. I think it is still a few years until both extensions open but it will probably hold the world record for the tram service with the most stops once it does.
Renfe's metre gauge Ferrol-Oviedo line has two end-to-end trains per day which make up to 103 stops.
I have just had a look at the timetable and i count a total of 104 stops including the origin station and destination station. I think this will probably easily be the world record holder for the most calls on one service.