The NFLPA updates their public salary cap report each day, and the latest update has the San Francisco 49ers holding $59,190,891 in cap space. They are a close second behind the Jacksonville Jaguars, who currently have $60,688,257 in cap space.
We know the 49ers have a ton of cap space, but it is worth noting the public salary cap report does not include every contract that has been signed thus far. After a player and team execute a contract, they then submit it to the NFLPA and the NFL Management Council. The latter is effectively the "management" side of the union/management relationship.