Anyone wanting validity for all stations would of course go for an
All Line Rover, but I presume that's not the point of this question?
I suspect to answer your question would require quite a bit of computing skills rather than routeing guide knowledge. If you purely wanted to buy point to point season tickets - rather than rangers or rovers - if nothing else you'd need tickets to cover pretty much every branch that only has one connection to the rest of the network, as through tickets wouldn't, with some exceptions, cover them. So looking at Scotland alone, you'd need tickets to cover:
* Kyle of Lochalsh to Dingwall
* Thurso to Georgemas Junction
* Wick to Georgemas Junction
* Stirling to Alloa
* North Berwick to Drem
* Oban to Crianlarich
* Mallaig to Crianlarich
* Helensburgh Central to Cardross
* Balloch to Darleoch
* Milngavie to Westerton
* Edinburgh to Tweedbank
* Lanark to Carluke
* Lakhall to Hamilton Central
* East Kilbride to Pollokshaws
* Neilston to Cathcart
* Paisley to Glasgow
* Gourock to Port Glasgow
* Wemyss Bay to Port Glasgow
* Stranraer to Troon
* Largs to Ardrossan South Beach
* Ardrossan Harbour to Ardrossan South Beach
And that's before you've started to try and cover through routes with a clever understanding of the routing guide so that one ticket covers multiple routes between stations.
Now you wouldn't actually need 1 per branch line - you could find ways of combining them together (eg Largs - Adrossan Stations would cover two branches; Drem - Tweedbank would cover two branches etc etc) but still even if you only need an average 1 per 2 branchlines, and I think that's optimistic, then that's at least 10 or 11 just to cover Scottish branchlines. Expand that notion out to the national network and look how many branches there are (eg South Wales!) and there's plenty more to cover!! I'd be surprised if the figure was less than 50 tickets for the country as a whole.