No reservations needed from Salzburg.

A reservation however is free of charge in 1st class and only €3 in 2nd when booked on tickets.oebb.at.

Reservations are not required but you could have to stand if the train is very busy and you don't have a reservation. I'd recommend to make them for peace of mind, 3€ through ÖBB with a seat map. :)

Saturdays are very busy so it's likely you'll have to stand otherwise. Of course no need to get them right now and you could wait until closer to travel date.

This is the same for most trains all across Europe, including in the UK.

Thanks for this information, I will reserve seats to be on the safe side

One more quick question. I will be coming from Bad Goisern on an early train on Saturday 27th to Salzburg ( before train to Budapest).  I’m guessing I really don’t need to reserve that one?! 

It's a regional train without reservations yes.

However coming from Bad Goisern Salzburg is a big detour, better go towards Linz. :)

Oh I didn’t realise that. I thought I needed to go Bad Goisern- Salzburg-Budapest 

Do you mean Bad Goisern- Linz-Salzburg? 
When I search Bad Goisern to Budapest it seems to go via Attnang-Puchheim and Vienna or Stainach-Irdning and Vienna. And a few warmings  of not enough time between trains?

Thank you so much for any information! 

The last one.

If you're looking on the Interrail website or in the rail planner app, then please never use those to plan. They're incomplete, often out of date and unreliable. National websites/apps are much more reliable. For International journeys, bahn.com, or in this case, fahrplan.oebb.at is a good choice.

Many thanks for all of your help.

I have just reserved seats using OBB from Bad Goisern via Wien HBf to Budapest. 
 

As you said it’s a better route