Hi, this question has been asked a few times in the past.

/u/Astrogator happens to have an answer to this question here, particularly on how when and where legionaries were located mattered towards their rates for survival, but also goes as far as breaking down average life expectancy in the Roman empire as well.

https://www.reddit.com/r/AskHistorians/comments/391ozi/how_high_was_the_chance_of_an_auxiliary_soldier/crzsanq/

As u/Gasmask134 has already answered part of your question regarding length of service and probable survival rates for a period of service, I'll tackle the rest of this.

Would there have been a level of success that a legion could have where the state would actually not be able to afford to provide land to the sheer amount of surviving veterans? Would the conquering of new lands have always balanced this out? Did the land grants have any influence on the amount of expansion and conquering that Rome was involved in?

If there was a strain put on the state in providing land would there have ever been an attempt by the Senate to throw specific veteran units into campaigns where they could be assured of having a number of troops killed, thereby lessening the burden?

So this question regards the legionary praemia and granting of land upon the end of a service term. Basically, after the so-called "Marian reforms" (which actually wasn't standardized, it was adopted on a case-by-case basis for each army raised in the late republic) the common practice was to settle "colonies" of veterans on newly occupied or conquered land to solve the issue of career soldiers that were retiring. Obviously as available land declined, this would become a problem, so Augustus solved this issue by establishing the praemia when he professionalized the Roman army under his reign. The praemia was a pension of funds given at the end of service, deducted from a soldiers' annual pay, from funds stored in the aerarium militare (the military treasury). According to Phang, Roman Military Service, this is estimated at 12,000 Sesterces under the reign of Augustus. The veteran could then use this to purchase land, but was no longer provided a land grant by default.

It should be noted that land was still granted in some cases, however. This could be recent acquisitions such as the establishment of colonies in Syria or Dacia, or the revitilization of agri deserti (owned lands that weren't producing and were a heavy tax burden). We see this case in the 5th century, for example, where the Alans of Goar and Sambida are granted agri deserti and "settled" as military veterans on top of the usual practice of "hospitalitas."

I'm not sure what time frame to put on this to get the best answers. Wikipedia mentions that the legions became mostly comprised of barabrian troops by the time of Diocletian. So I think I should specify from the time of the reforms (105BC to circa 200AD.

This is also a common misconception and I'm sure there's a FAQ on the BS that is "barbarization" elsewhere. In 1992 Hugh Elton produced a study which showed roughly 3/4 of the 4th-5th century Roman army was composed of recruits originating from within the Roman empire. The Romans had always recruited "barbarians" and they were no less reliable than the average Roman soldier.

Alrighty SOOOO. There's a fair bit to unpack here, bear with me and I'll see what I can do. First off, let's discuss the military reforms of the Roman military (gonna use this word instead of army, army is a weird word in this context because everyone's rules for their own army were different for a little while) in the first century BCE (and slightly before but). So in the mid-second century BCE, the Roman empire was having a small problem. Its military, which had been built up as a civilian military and not at all as a standing army, had beaten (and conquered) enormous swathes of land, from Spain to Greece, and areas even beyond these had declared themselves as "friends" of Rome. Men who were nominally supposed to be recruited only for one season of war at a time were now being held for multiple years at a time, both in the field and in a more "active" military capacity (activity changes based on location, please see local proconsul for details). New positions were created ad hoc to try to handle this situation which the Roman government was never meant to handle in any capacity (see the aforementioned proconsulships - governors for provinces, since Rome now had too many provinces to just let the regular consuls handle things).

The Romans stationed here were now held away from their homes and families for years at a time - and, while Roman women did plenty of farmwork themselves,^1 they couldn't manage an entire estate alone. Especially not with the rise of the latifundia: large, cobbled-together estates of land, worked by the masses of slaves coming in from successful wars, that were technically not exactly legal, but their owners were wealthy enough that the laws weren't really enforced. So these soldiers, upon coming home, were left with a broke family and land that was gone to seed. It was super easy for them to just sell the land to a buyer (who was pressuring them anyway, and offered a...reasonableish price) and make their way to Rome, where they would attempt to find work. Except that they often couldn't and, as the numbers of landless poor swelled, the number of landed farmers - who were ostensibly the only ones "legally" allowed in the Roman army - diminished. There are records of an old veteran who re-enrolled in the Roman military for 22 years during the Macedonian Wars (mid second century).^2 That's unusual as heck, and just goes to show that there was no work for these guys. The rules around who was allowed to be in the military were....bent. And then bent, and then bent, and then bent. As available "legal" manpower kept dropping, so the rules kept relaxing, until Marius became the straw to break the camel's back - he recruited directly from these masses of (now) landless men in Rome, who just wanted a job. He offered them a salary, a share of any loot they got, and, most importantly, a chance to start again with plots of land.

This land was not contingent on length of their term. It was contingent on the successful completion of his campaign(s). Now, "legally," a consul was only going to be in office - and therefore in command - for one year, but those rules at this point in time were just so much tradition that was being handwaved away because reasons. Marius was elected consul to deal with the Jugurthan War. Which he successfully brought to a close (thanks to an enterprising young lieutenant named Sulla). But then, a terrifying thing happened to happen! Invasion! Well, sort of. Rumours of invaders from beyond the Alps, terrifying barbarians who crushed everything in their path (including 3 full Roman armies). Rome was scared. And therefore, scared people did what scared people do, ignored tradition, and elected Marius as consul again (there was supposed to be a ten year interval but). And again, and again, and again, and again. Six times, Marius was elected consul. He beat back the barbarians, he settled his veterans on some prime land in Africa (seriously some of the best agricultural territory in the Roman world), and all was...well, not well. But that's another story. We're talking about vets here.

Theoretically (heh, "rules" in the 1st century BCE), Romans during the Republic were only required to serve for sixteen seasons/years. You could re-enroll more often if you wanted, but sixteen was the theoretical max. With the rise of the "Marian Legions" (gonna use this for ease of writing, just remember that every general basically made up their own rules in this period), that "sixteen year" rule turned into a "for the duration of the campaign, which will probably be fewer than 16 years" rule. However that started causing issues, particularly around the middle of the first century BCE, but I'll get to that in a sec. Your question is about how many soldiers would make it past this campaign period, and how many could the state (as it were) afford. Well, there's good and bad news here. Bad news first, I can't give you exact numbers, though Caesar's (Julius Caesar, the famous one of getting-stabbed-by-the-Senate-in-44-BCE fame) numbers are closer than nothing (while some of his older legions were HEAVILY depleted - 50% or fewer, to the point where he had to merge a couple), and most of his men did survive.^3 The state didn't have to worry too much about settling these guys, though - after their initial settlement, the Senate essentially ignored them.^4 [This footnote is a rather huge one that describes veteran settlement through both the Republic and the first century BCE. It’s a bit non sequitur here, which is why it’s footnoted, but if you're interested in veteran colonization in these time periods please, read it and feel free to ask further questions!]