I don’t know about any limit here. Every DT user can create own dashboards, clone existing ones and you as admin even don’t know about it. It can be right now checked only via configuration api. Making limit here would be painful because you don’t know in UI how many Dashboard are already created and who is their owner (if they are not public).
There is limit in number of tiles that can be on single dashboard,
at the moment there is no limit, but soon there will be limit of 2k of dashboards / environment. I doubt that a limit of 2k will be painful, as this is really a huge number. I can see more the management aspect of it, where we also have plans for it, like tags, better filtering etc.