Just to add to what Jim said and to clarify a
bit further
as far as on board storage I agree I would go for the 1TB
What Jim is talking about is recording to an external Drive which stores the audio on the external drive (which is viable)
What you are talking about is recording to your boot drive (mac HD) which stores the audio on you boot drive and then moving the finished songs or group/album from the boot drive to an external drive (which is also viable ). And is what I do with my video files..... I do however leave my audio productions on my boot drive
So in order to get an idea of how much storage your songs will use,
is very simple (if your songs are in their own folders ?) ---Right click on the folder for your song and choose "Get Info" it will tell you how much in either megabytes or gigabytes for bigger productions they are, then just calculate times the number of of songs you think you will likely produce in say a year.
OR if all your songs are currently stored on your boot drive ( Macintosh HD). just right click it and "Get Info" and that will give the total drive space you are currently using--- Apps and music files ,,,
For example I keep all my protools session folders in a single parent folder on my desk top labeled "Kev's PT sessions" the entire folder of aprox. 100 songs with audio files plus 9 session templates (no audio files ) is about 45.5 GB total
Most likely (depending on the number of applications you have on your boot drive) they will take up any where from 200 to 300 GB for just them
So taking the higher figure to be safe of 300 . On a 1 TB drive minus 300 GB leaves aprox 700 GB of file storage.. So take that 700 GB divided by the 45.5 GB my 100 songs take up as an example ,,,, equals 15.38. so that means I could likely store 1,538 songs on a 1 TB drive before using it up.
But be aware video files are an entirely different situation
On my boot drive currently I have about 9 video files of about 3:00 minute length, mostly in 4K and they take up 338.75 GB So I will be moving them soon