From where to book train tickets

1) If you already have a Swiss pass, you are covered not just to Zürich, but as far as the last station in Switzerland. For the trains from Zürich to Innsbruck that is Buchs.
See the map which shows where the Swiss Pass is valid here: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich_en.pdf
You should get a copy of this map with your pass.

2) You then need to buy a ticket from Buchs to Innsbruck. The first place to go to buy tickets should always be the railway company running the trains. In this case ÖBB (Austrian Federal Railways): http://www.oebb.at/en/
An alternative is the SBB (Swiss Federal Railways) site: https://www.sbb.ch/en/

3) I am not sure there is a limit. You can buy it from the SBB website: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-travel-pass.html
You can get a print-at-home ticket.

4) The only trains you need to make reservations on in Switzerland are the 4 tourist extra trains (Glacier Express, Bernina Express etc.). On most trains reservations are not even possible. There is at least one train per hour on all Swiss routes. It is designed to be a "turn-up-and-go" service,

3.Also how early can we buy the Swiss Travel Pass ?
--The Swiss Travel Pass is valid up to 6 months before travel.

small addition to Chris' advice. There are more than one Buchs in Switzerland, the one you need is usually written as Buchs SG (with nothing else) in timetable and fare systems. Any longer name will be a bus stop and Buchs AG is a different place! The direct trains from Zurich to Innsbruck stop there but not long enough for you to jump out and buy an onward ticket. OEBB website currently offering advance booking tickets from €9 !!!!

Thanks Chris if you hadnt let me know I was going to buy the ticket from Zurich onwards.
Linda - Thank you , I didnt want to put off buying the pass till the last minute.
Steve - Thank you . I did see a lot of Buchs and I did get confused .

I just have a few more questions-
1 . So I need to buy a train ticket that starts from Buchs SG say at 11.54 and reaches at Innsbruck at 14:11 . My initial point of entry is Zurich , So I just get on the train that starts at 10.40 from Zurich(scheduled to reach Innsbruck at 14:11) and the rest of the journey is covered from Buchs SG to Innsbruck . Am I right ?
2. Is Innsbruck Hbf the destination I have to search for ?
3. If I were to miss the train by any chance , would I need to get a new ticket from the station ? I am just worried about getting on the wrong train since I am buying the ticket in advance .
4. After buying the Swiss Travel Pass ,when would the pass get activated ?

1) Yes, you buy a ticket on the train that leaves Buchs SG at 11:54. Yes, the train that leaves Zurich at 10:40 is the one you get on.
2) Yes Innsbruck Hbf is the main station in Innsbruck. Hbf is the abbreviation for Hauptbahnhof, the German word for "main train station".
3) If you buy a savings fare ticket, you must ride the train the ticket is for. If you get on the wrong train, you will have to buy another ticket. It is not likely you will get on the wrong train. The train originates at Zurich Hbf and will be sitting on its assigned track for at least 15 minutes before departing. The train will be shown on the big overhead signboard in the main hall. It will also be on an electric sign at the head of the platform. Its final destination is Budapest. It will also look like this: https://www.eurail.com/sites/eurail.com/files/styles/asset_image_images_slider_big/public/railjet_high-speed_train_at_train_station_salzburg_austria.jpg?itok=E7HngzPX
Notice the color and the big letters on the side of all the cars, "railjet"
You will also see train personel on the platform standing by the train. Just show your ticket to one of them and confirm it is the right train.
4) If you buy the pass on-line, you will indicate with the purchase the dates it is active. If you buy the pass when you arrive in Switzerland, just tell the agent when you want the pass to begin, probably the day you buy it.

Thanks Sam for the quick reply.

You can buy the ticket from Zurich to Innsbruck on SBB and choose a reduction/ credit for your Swiss Pass. Select "GA 2 Kl" ( assuming you are traveling 2d class) and it will show the price as 32 CHF instead of the full price of 78 CHF. This ticket will bee for the specific train you choose, such as the 10:40 from Zurich HBf.

But it looks like a possibility of getting a 9 EUR Sparschiene ticket from Buchs SG to Innsbruck.

as said yesterday, and from OEBB not SBB who seem to be charging full fare.

I think I would wait until I got to Switzerland to worry about it. I got my half-price ticket and ticket to Lucerne at the SBB office at the Zurich airport. Took about 10 minutes.