Is there a limit on a number of words in a dictionary?

Yes, there is a limit. The size of a single global custom dictionary for the Cloud version is limited to two (2) million symbols including spaces and commas.

If you are using the on-premise/server version v5.23.0.0+, one single dictionary is limited to up 5MB. Previously there was not such limit, but creating big dictionaries leads to bad performance indicators.