The ASA 5545-X supports 2500 VPN tunnels, reference here. Even though you only have 1 IP address, the sequence number used on the crypto map is used to distinguish between the different peers. So I don't see why in your scenario you could not configure up to 2500 VPN tunnels.
Sessions is generally used to refer to remote access VPN. Since almost nobody still uses the old IPsec IKEv1, we generally equate sessions with SSL VPN sessions for remote access clients. In that case there is a separate limit and you require AnyConnect licenses for your sessions. In the case of SSL VPN, the limit is also 2500.
So you could have up to 2500 SSL VPN sessions (assuming licenses are installed) in addition to 2500 IPsec site-to-site VPNs concurrently active.