There's no defined limit after which you will receive an error message saying you've exhausted your quota of =TR functions. This however does not mean you can expect to reliably retrieve unlimited amounts of data. As you increase your data consumption at some point something somewhere will become and bottleneck and you will start seeing some sort of unexpected behavior. It's impossible to say at what point this will happen and what exact symptoms you will experience, as there are way too many variables in play. Think of it this way. There's no defined limit on the number of tabs you can open in Chrome browser. But this doesn't mean you can expect to be able to open 5 million of them. When no limit is defined there's always a practical limit of sorts.
It is generally, albeit not universally, more efficient to use a single instance of =TR function for multiple securities and fields over a separate instance of the function for each security/field pair.

I don't think I understand the question. Would you care to elaborate? It would be very helpful if you could describe your use case in detail and what challenges you're facing and trying to solve.

Sure, I aggregating Thomson data in excel, using queries like: '=TR("',securityname,'",','"TR.Whatever")'

I do this for multiple securities and many different factors. I do this instead of using the screen + metrics -> export because I use a lot of metrics and this gives me more control. So, I construct these excel files then populate them with the Thomson excel plugin. I want to know how many of these queries I can use in a single excel file.