Patricians were the direct male line descendants of the original Senate, the Patres, the "fathers" of Rome. By the Late Republic most of the families had died out over the years. Of those remaining some were still in power, often possessing lots of property and political clout having from five centuries worth of powerful ancestors, while others weren't and had fell on hard times over the years (like Sulla and Caesar in their youths).

Everyone who wasn't a Patrician, wasn't a direct male line descendant of the original Senate were Plebeians, the Roman People. Those could be poor Roman proletarii. Or they could be ex-slaves given their freedom. Or recently enfranchised former member of a Socii community from the Latin or Italian city state gifted with citizenship. It could mean a Roman so poor their property wasn't even bothered to be tallied for worth during a Census. Or it could mean a Roman plutocrat and member of the Equestrian Order, the richest of the Roman citizens, who might be a massive landowner, a merchant, a banker all in one. Or lastly, it could be an extremely powerful member of a noble Senatorial family of one of the few families regularly producing consuls, such as the Caecilii Metellii, who were dominant members of the oligarchy controlling Rome. So the word is not synonymous with being poor or without power.

Rome during time period in question conducted a census every five years (if possible), where former consuls were elected as Censors, and along with an army of bureaucrats, went and tallied up the total number of Roman citizens, as well as their property worth. How much someone was worth was used to place them into voting centuries of the Centuriate Assembly, the Comitia Centuriata, designated Class I through Class V. Those who had so little wealth that they could not qualify for Class V were the proletarii, who were also derided as the Capite Censi, the Head Count, as only their total numbers were tallied and not their property. They had voting rights, but their power voting in the centuriate assembly was essentially nil.

The more wealth one possessed, and the older they were, the stronger their century vote, as the complicated Roman voting system using the centuriate assembly allowed the higher classes to vote first, and the seniors (47 and older) in each century to vote before the junior (17-46). The system is very confusing and if you want more info on it you can read about it here.

Simplifying it for the sake of Reddit, there were 193 voting centuries, a majority was needed to win a vote. The Equestrian Order had 18 centuries, Class I had another 80 centuries. If they all voted together (which often happened), then even calling Class II wasn't necessary. If a majority wasn't received by that point, more centuries from more classes would vote until finally a majority came about. However, that would always happen before a member of the Proletarii/Capite Censi could vote, and thus they had no ability to influence an election based on the Centuriate Assembly (consuls, praetors, censors).

There was another manner of voting, using Tribes, of which there were 35. When it came to tribal elections, which were also used for Plebiscites (voting done by the Plebeian Assembly, no Patricians allowed), all 35 tribes voted, and the majority of tribes won. However, they were not distributed equally. Some tribes had low numbers of citizens, while the five urban tribes had the bulk of the population, including the poor, recently enfranchised Socii, and freedmen. So in this situation too, because they had no power, the Proletarii/Capite Censi could not have their vote counted sufficiently.

Where they did have power was not being raised up from military service, but by being won over by various Populares, politicians who favored using the Plebeians for power, and not favoring the Oligarchy members of the Senate. They used the poorer Romans either for spontaneous Plebiscites, quickly arranged votes held by the Plebeian Assembly that would favor those who were present, or worse, for mob intimidation and violence (such as that made famous by Clodius). Rome the city's population was over 2 million by the time of the mid 1st Cent BC, and the vast majority in it were poor and ex-slaves. Many of Rome's citizens didn't actually live in Rome itself, they lived all over Italy the peninsula as well as in provinces, and couldn't be travel there for every election. Because Plebiscites were often unscheduled, spur of the moment affairs, it became imperative for the Populares to win over those inside Rome itself, not only the poor but obviously the other citizens living in the city.

Marius did not make the Proletarii the backbone of the Legions, nor were they ever really the backbone. Marius never actually recruited an army, once in his career he recruited a single Supplementum in 107 BC, a replacement draft used to fill up an existing army with replacements to make up for casualties/desertions. After his victory in 107 BC, to include taking "stealing" the command of the Jugurthine War from the powerful Plebeian Consular Q. Caecilius Metellus, a vengeful/angry Senate denied Marius the ability to raise a brand new army, as well as denying him the ability to even hold a fully sponsored Supplementum that would have forced Romans to show up to be conscripted. Instead, Marius was forced to call on volunteers, from all classes, which he enlisted roughly 5,000 Roman citizens, and did so with no regards to property. That's it, that is the nature of Marius' involvement with the Capite Censi.

How many did he enlist? Most historians don't think it was that many. Marius was very popular among all Romans outside the Senate. His chief backers weren't the poor, it was the Equestrian Order and the wealthy of Class I-V, who not only got him elected as Consul in 107 BC despite barely even campaigning (he showed up late), who also got him elected consul every year from 104-100 BC. So the majority of the volunteers who showed up to Marius' volunteer supplementum were likely propertied Romans.