(1) With my current hub, I have some 80 Zwave devices, plus a few Wifi devices. As each Zwave devices has several nodes, is there a capacity limit with Hubitat ?
(2) Furthermore, does Hubitat use Zwave series 700 dongle and/or can I attach a separate Series 700 dongle ?
The C7 uses the 700 series chipset. I think the zwave radio tops out at around 256 with 6 being reserved for internal.
I have more than 80 devices running on my HE with no issues other than regular zwave care and feeding.
No, basically.
The C-7 has a 700 chip inside but does not accept dongles.
The C-3, C-4 require a region specific 500 series dongle BUT not the 700 series dongle.
The C-5 has an internal 500 series chip for the US and supports an external dongle for other regions, but not the 700 series dongle.
To add to the above, while the hub can theoretically support just under 256 Z-Wave devices [EDIT: 232), as noted, I think I've seen posts suggesting that networks of more than about 40-50 devices may require particular care (minimizing the use of S0, eliminating as much unnecessary traffic like power reports as possible, etc.--all good ideas regardless but perhaps particularly important as you get larger). This will likely depend on specifics of your environment, and as you can see above, more is certainly possible--but this may be worth keeping in mind.
Beyond that, Z-Wave itself has a limit of four intermediate "hops" between nodes, so you could eventually reach some maximum range. Most devices should find and use good routes automatically (pre-Plus devices might require some love), and if you haven't noticed any issues so far, you're probably good. 
232 is the max in the non-LR protocol.
Thanks. Sorry to be specific, 80 Zwave devices are ok, even if they come with additional nodes (Scene etc) ?
To explain my question... I currently use the excellent ISY 994 / Polisy hub set-up, but I have heard about Hubitat and am trying to understand more of what it is capable.
Over time I may transition from Zwave to Lutron RA3.
I'm not sure what you mean with "nodes" here. Each Z-Wave device is a node: your hub, a switch/dimmer/outlet/sensor, etc., and each is assigned a node ID by the hub/controller. (This is part of where the device limit comes from: node IDs are two-digit hex numbers, with 0x00 and 0xFF being the lowest and highest possible values. ID 0x00 is not used, 0xFF is 255, and some are reserved, bringing the total to...I guess 232 as JasonJoel notes.) Each device gets only one node ID.
Maybe you mean "endpoints" or "channels," like a dual smart plug with one endpoint for each outlet? If so, this doesn't count more towards your device limit--it's just the node IDs that matter. Or perhaps you mean "command classes"? If so, to my knowledge, there is no limit on a network to the total number of supported command classes across all your devices on the network. The only practical concerns I think of come back to the above: some of these might correspond to potentially "chatty"/frequent commands, like power reporting. For such devices, you may wish to consider either disabling this feature or toning it down to just the quantity of reports you need for whatever automations you're using this data for.
As for the "80 devices" thing, theoretically, you are good. But nobody can speak to the practical details of your own network without knowing more information. There are certainly people who have networks that large without problem--and I'd guess you're one of them since I don't see any complaints. 
Download the Hubitat app
