How many mountain top trips?

To me, at least, each mountain-top excursion is unique. For example, going up to the Jungfraujoch is quite different than going up to the Schilthorn. From transportation method to the view, each has its own unique character. I would certainly go up at least one mountain in each region (one in the Berner Oberland, one in Zermatt) because the views will be drastically different. If you have the time and inclination, going up more mountains can certainly be worthwhile. Be flexible, too. If the day is very cloudy, going up to a summit can be quite disappointing. I would be particularly cautious with the journey to the Jungfraujoch. It's quite expensive, and if it's cloudy, you won't see a thing from the top.

At least go to see the high peaks of the Pennine Alps around Zermatt. Although the view from the valley floor isn't as immediately stunning as the Lauterbrunnen Valley (apart from the Matterhorn, if it's visible), once you ascend past the blocking view of the lower slopes, the jagged summits towering above you are a sight like no other. I would prioritize this first.

But as the other poster mentioned, you need to have a certain amount of build-in flexibility if the weather doesn't cooperate. Plan at least two full days at each Alpine destination, and prepare to lose some to rain or cloud cover. So, it would probably be a better plan to spend more time at fewer locations rather than less time at more locations. If you only budget one full day at a particular Alpine region, if the weather wipes out that day, that excursion will be a complete waste. Especially in an expensive country like Switzerland.

How many is enough? depends on these factors:

1) weather
2) cost
3) whether you will regret not going

On my trip to Switzerland we did two mountain trips. The Gornergrat out of Zermatt with its stupendous views of the Matterhorn and the Pennine Alps was, as Tom said, an experience I would not want to have missed. The other one was up Mt Pilatus out of Luzern and it was a totally different experience because of the view and also getting there, we took the gondola/aerial cableway up on one side and the incline train down the other and that was different. So I think two mountains with two very different experiences is definitely not too much.

I must say that I loved both and am glad I did both. If the option for a 3rd would have presented itself I'm sure I would have gone up there too. So, I agree with Stan's three factors as being the deciders, they make a lot of sense.

My husband and I spent 12 days in Switzerland Aug-Sept 2013 to celebrate our 60th birthdays.
We did Lauterbrunnen/Jungfro/Berner Oberland area, hiked from pension/hut to pension/hut near Zermatt with outstanding views of the Matterhorn and included the Appenzell area for two nights. We loved all three uniquely different areas. We rented a car and enjoying seeing the small towns and sights between each location. We hiked(the swiss can cover the same trail in 1/2 the time!) around 25 miles total and have to say our favorite stays were in the pension/huts that you have to hike/take a lift to. Worth the effort! (Dorms are an experience!) If you have the time and since you are buying the ticket to get there…the more you can take in the better. Of course the majority of our lodging was found via Rick Steve's book..if you want hotel/pension suggestions, I would be happy to share where we stayed. However, I did find Bookings.com to be a great help as well. Happy travels.

I cannot get enough of Swiss mountain tops! I probably have been up 30 of them and still want to go back for more...each view is unique, different, and more spectacular than the last! Like the previous posters have noted though, weather will be a factor in early fall. Allow a decent window of time so you won't be disappointed. I highly recommend going to Kandersteg...it doesn't get much attention here but it's a UNESCO World Heritage Site and truly can't be beat in the BO.