To set a limit on the number of allowed IPSEC VPN session on an Cisco ASA 5540 we need to define how many sessions \ users are allowed to be connected to the ASA in each given time.
By default the number of allowed VPN session is unlimited.
To set a limit we need use the Cisco ASDM.
Once logged to the ASDM go to:
Configuration > remote access VPN > Network (client) access > advanced > IPsec > System options
Once there change the maximum IPsec sessions, to the applicable number.