How can I find my train platform number at Zurich?

According to www.sbb.ch, on the exact date of 2/Aug/2020 your train from Paris (16:21h TGV from Gare de Lyon) arrives at Zürich platform 18 at 20:26h, as you state.

The Deutsche Bahn website (bahn.de) lists your 20:40h train to Vienna as leaving from platform 8. Note that this train (NJ465) stops in Feldkirch (platform 2) and then you have to transfer to NJ447 to Vienna (platform 3).

For some reason, the SBB don't seem to think this is a viable route to get to Vienna and so don't list it if you ask Zürich-Vienna.

Double-check the details on your ticket itinerary - there are quite a few options and it is a bit complicated.

Sorry if this answer has already an accepted answer, but unfortunately it needs a major clarification

Is there any site where I can find full details of the platform numbers in advance

Generally not, and depends on how in advance. Train platforms are similar but not identical in concept and management to airport gates. It's a lot less bulky for a train company to get a platform at a station than an airline to get a gate (and all required staff) for a flight.

This because tracks are assigned really shortly to train's arrival at the station, for a variety of reasons. Railway traffic control has a few constraints over the choice of the track/platform to assign to a train:

• Electrification: a train not running a HSL (25000V AC) must be routed to a regular line (3000V DC)
• Length of the train: a fully-featured two-stock ICE can't enter a half-track in a terminal station, which is used for regional traffic
• Availability of outbound track: in my experience, this often results in long-running trains stopping at the middle in the middle of a large station (e.g. it comes from the line on "your left" and will proceed to "your right", to simplify a lot)

Indeed, if you connect to SBB's real time traffic, you can get realtime data

And no, you can't know long before departure, unlike airport gates.

to avoid getting lost at the station

Getting lost at Zurich HB is a hard work. Once you get off the train, find your numbered platform, head to the head of the platform and look for the timetable, which will likely indicate your next platform given the short layover.

Kudos to @jcaron's comment, reported verbatim

Most internal trains in Switzerland won’t wait for delayed passengers as the regular scheduling usually means you can get another train within the hour, sometimes much less. However the last train of the day on a given route is usually an exception. Make sure you advise the train master of your connection if your train runs late