There is no specification published for the number of entries allowed in the 3CX phonebook, but my guess is that it is pretty much unlimited excepting the disk size limitation and any limitation the database engine itself may have).
Most phones have limitations, but this is mostly a design consideration of the manufacturer as to the amount of space they want to make available as space is memory and memory costs $.
The model of Aastra phones that I have used, all download the phone book, so I am guessing it likely that whatever model you have does the same. Therefore if 3CX has more than 200 entries, your phone will be impacted.
Unfortunately, I do not know if the impact will be the phone only taking the first 200 entries or, if the phone expects to see some kind of eof (end of file) message and failing receiving that will assume that the download is corrupted and simply not import anything.
I do not have any Aastras in an installation that big.