Disk Capacity 500 gb falls to 250 gb after deleting files
You have one SSD (disk0) with 251 GB containing one visible volume 'Macintosh HD' (disk0s2) with a capacity of 250.14 GB of which ~54.3 GB are used and 195.88 GB should be free.

Additionally you have 2 devices attached:


probably a thumb drive (disk1) with one volume named 'NO NAME' (disk1s1) and the size 8 GB
a Canon SD-Card (in an attached camera?) (disk2)  with one volume named 'CANON_DC' (disk2s1) and the size 8 GB


If the Finder doesn't show the full free space (195.88 GB) for 'Macintosh HD' or still complains about an 'almost full startup disk' check several answers here at ase:

Why is my startup disk so full?
Finder Reports Disk is Full, Other Tools Don't Account for it
How can I figure out what's slowly eating my HD space?