I think you might be misinterpreting Elon's tweet. The $500m will be spent "on new sites and expansions", which means "on new sites and existing sites". Quite possibly a majority of the $500m will be spent on existing sites, and my guess is that many of the new sites would be large ones (>12 chargers), so there may not be that many of them.
Tesla's cost per charger has been estimated to be
about $43,000, so a 12-charger installation is roughly $500k in charger costs, implying that naively the $500 million could buy about 12,000 chargers. But the charger cost is likely a minority of the costs involved in actually constructing (or expanding) a site: there's land acquisition, permitting, utility hookup, costs for building out more power to the location, paving, solar canopies, megapacks for onsite storage capacity, etc., etc.
So this $500m spending figure could still represent a signifcant slowdown in the overall pace of Supercharger buildout. (And a third of the year is already gone, even before the layoffs.) So I stand by my assessment that Elon's firing of the entire SC team was a terrible, verging on catastrophic, unforced error; one that Tesla will come to heavily regret.