Surround set up always requires two speakers. 

Thank you for the response. Considering the price of the Sono Era 300, is Sono Era 100 also good with the  Sonos Arc Ultra for Surround or ERA 300 is the best considering long term. RooM Size is 25.11 M2

I have tried both and if you have the space to properly place the 300s get them. The 100s are not bad by any means but the 300s, particularly on Atmos content are fantastic.

Thank you very much for response.

As has been said you need two speakers to create surround effects.

The Arc Ultra is designed to produce an Immersive effect when watching movies with Dolby Atmos sound tracks and/or music recorded in Dolby Atmos. It also does equally well with Dolby Digital 5.1 sound. 

The main difference between the Era 300 and Era 100 is that the latter is a stereo speaker. The Era 300 is a Dolby Atmos; as well as a stereo speaker. 

Using the Era 100’s as surrounds the Dolby Atmos effects will all be processed by the Arc Ultra. You’ll still get an immersive effect with the Era 100’s; but not FULL immersion.

Using the Era 300’s you’ll get FULL Dolby Atmos immersion as they will out-put Dolby Atmos effects that are contained in the surround channels of a Dolby Atmos movie thus complementing the Arc Ultra.

Bottom-line

• If you watch a lot of Dolby Atmos movies typically via movie streaming services, AppleTV and streaming sticks, the Era 300’s are the best choice.
• If you watch mostly cable TV which is typically stereo and/or Dolby Digital 5.1 the Era 100’s will serve you well.

Amazon Music also offers Atmos when played on the TV using the Firestick App.

Amazing sound with just the Arc, or with 300s or 100s.

Other sources might also offer something but I don't use them to know.

Apple Music offers Dolby Atmos whether you use the Sonos app for playback, or the Apple Music app on the TV or Apple TV box, or Airplay from the Apple Music app on a device.

Thank you I currently use Apple TV .however do notice sometimes the Sonos app doesn’t show the atmos sign in the phone for some reason

Because not every track is in Atmos. Many are but a great many more are not. You’ll see Lossless for those. 

This issue is more when watching movies the same series shows as atmos some times in app and when  i watch after some time it doesn’t or shows Dolby digital plus

The Arc can only play what is sent to it, so if Atmos is working some of the time then Atmos is working. Any other sound format you’re seeing for the same series will be down to the streaming service or broadcaster for those particular episodes.

(To correct myself on one point from earlier - there is no Dolby Atmos when using Airplay to the Sonos speakers.)

Some services decide what to stream based on the connection quality. That can vary moment to moment as other wifi and network users near you do different things.

Does Sonos Arc Ultra  with just 2 Sonos Eras would work fine for 25 M2 room  or is Sub 4 still recommended

It will work, adding the Sub is a matter of musical taste.

That said I have a Sub in a 9 m2 room and love it, but I have Ones and Beams in other rooms that I find sufficient for what they are used for.

Start with the Arc, add a Sub if you aren't satisfied, maybe add a second. There is a feature request here somewhere to allow four Subs, chip in there if two doesn't satisfy.