"The huge number of new esperantists makes me happy."
Translation:La grandega nombro da novaj esperantistoj feliĉigas min.
I'm going to run this one by the experts. "Nombro" is always tricky as to whether it takes "da".
Da shows quantity, so if "nombro" is used as a quantity, it needs "da". You know it's a quantity in this sense if it answers the question "how many"
- Granda nombro da homoj lernas Esperanton. (How many learn? A great number learn.)
If "nombro" is not used as a quantity, then it's "de".
- La nombro de lernantoj estas granda. (What kind of number is big?)
My sense here is that this sentence is of the second kind and so should be "de"