If there is no 2020 season it is likely it is likely the owners concede to letting it count as a year in exchange for other concessions so it would not be that good of a situation for the Cowboys. After all for the owners it is not money out of their pocket and they would be more focused on trying to reduce what they had to pay out in a no football season.
As for the total number of times you can franchise a player, the technical number is 3, but the problem is that while this years tag is 31m, next years tag will be 20% more than that (higher in the unlikely event the top 5 average salaries goes over that) or 37m. 37 milion is fine but year 3 it jumps by 40% to a little over 50m which is not really sustainable. It is not even really affordable to even try putting the tag on a QB for that 3rd season. So basically Dak is on a 2 year 68.5 million dollar deal at which point either the Cowboys will have resigned him or he will get some mega deal from one of the, up to a dozen, teams that will be bidding for his services.