How many people could lightning network support at the moment?
It depends on the type of usage that dominates. Most of it is really nominal testing of the network atm (e.g. the graffiti wall website), but there is a some genuine commerce happening for VPN subscriptions and mobile topup (lightning is the 2nd most popular payment method at bitrefill.com, according to them).
Assuming enough bitcoin went into the system
Enough for what? Lightning payments are capped at ~ 0.04 BTC, the use-cases for that are limited (i.e. the previously impractical micro-payments use case). This will change in future, but this is what's possible today.
There is currently ~ 650 BTC in public lightning channels. It's not very easy to say how many users that can support until we know how it will be used. It's been said that the ultimate carrying capacity of lightning is many multiples higher than e.g. the VISA network, but that will simply incentivise the use of that extras capacity (new business models will almost certainly develop to take advantage of the novel capacity dynamics)
