Are you talking about increasing green ward or pink ward limit?
If you're talking about increasing the green ward limit, I think that would undo the effort put into the season 4 changes that aimed to shift supports away from the role of "ward dispenser". Riot wants vision to be a team effort. Sightstone is a cheap pickup that, really, anybody could buy, but supports often end up buying it because they generally don't scale as hard with gold, other lanes have more uses for the item slot and extra gold.
If we increase the green ward limit by one on ruby sightstone, not only does it force supports to go for it (as we need vision, and they already have the sightstone), it would require us to even stock up on an extra green ward or two, to keep up with having four wards down at once, plus all the scanning that goes on. Sightstone is meant to be an item which puts an end to ward buying. If you own a sightstone, you should not need to buy another green ward that game. Having a four ward limit late game, and trying to keep up with making use of that extra limit means we have to buy at least one or two extra green wards - this is bad.
If you're talking about increasing the pink ward limit, it would be a kind of niche upgrade (if they have lots of stealth then it might be useful), but again, it's forcing supports to make use of that two ward limit. Pink wards are fairly expensive, and if we choose to invest another 475g into a pink trinket to make use of the extra limit, we're sacrificing scanning for wards every minute. It would leave a hefty hole in the support's not-so-deep pockets.
One minor buff to a ruby sightstone is something I wouldn't mind seeing - assuming it doesn't become a mandatory rush. I'd love to see an upgrade that becomes more relevant late game, so you purchase the sightstone early game, and upgrade it again at 30+ minutes.
Because of how quickly wards can be killed late game (it's pretty hard to even put one down as an ADC can even kill it before it stealths), I propose this ruby sightstone buff.
UNIQUE passive: Ward Reinforcement - Wards dispensed by the ruby sightstone require six (rather than three) hits to be destroyed.
This helps give a ward a fighting chance when it's put down late game, and actually forces the enemy team to use a pink/scanner to kill the ward. It's also not a super-strong buff, meaning it doesn't make ruby sightstone a force rush, and some people might not ever even pick it up. It does give it a niche for late game, which I think is the perfect little nudge it needs.
Edit: Quick concern that Riot Baconhawk brought up with having wards at six health, scanning a ward with a tier 2 scanner generally gives a lone support time for five auto attacks. That means that a support with low base AS and no AA Reset (LEONAAAAAA) would not be able to kill a ruby sightstone ward in the time that it was scanned.
To sort of fix this while also solving another issue, it might be cool if all wards affected by a scanner instantly take one point of damage. As it stands, if a support scans a bush that has two wards, and they lack AS /AA reset, they can only do 5 hits, leaving one ward with one health. This would not only allow the reinforced ward to be killed by base AS in one scan, it would also help the lone support and reward good scan placement.
A less intrusive solution would be to have the passive provide a temporary shield, which is removed once the ward is stealthed.