802.1Q frame has a 12 Bits VLAN ID which translate to maximum possible 4096 vlans on a trunk.
ISL frame has a 15 Bits VLAN ID. based on this I would assumed ISL can support more than 4096 vlans. however, I'm being told only up to 1024 vlans are supported on a ISL trunk. is this true? if so, is there something in the ISL frame I can use to calculate the max vlan?
original ISL specification supported only 1024 vlans but later a revision of the protocol allows for support of 4096 vlans like it happens for 802.1Q.
the vlan-id field has been 15 bits wide since first version of ISL.
You need also to take in account that ISL has more overhead and that 802.1Q is standard based.
Your lab results look like correct.
to be noted that VTP is still limited to vlans 1-1024 in version 2 and vtp mode transparent is needed to use the extended range
Hope to help