I use a external AD database for vpn authentication login.
I just noticed by default, asa only allow 3 simultaneous login per account. I tried to login vpn using multiple PCs with same username, but only can login with 3 PCs, as long as I tried fourth one, one of the precious 3 got disconnect..
Is any way I can increase the number?
Yes, go into Group-policy mode and set the number to your desired setup:
group-policy WHATEVER general-atributes