a Canon EOS T7i camera with an EFS 18-55 zoom lens. I found a 70-300mm lens...can any one tell me how much more magnification I will get with this lens.
Short answer: lots. 😀
The one you have is, with that camera, a bit longer than a "standard" lens. The sensor in the camera is a bit smaller than the picture area of 35 mm. film. Maybe you knew that already. Because the sensor is smaller, a given focal length is more "telephoto" than it would be on a 35 mm. camera.
This link you may have already read. It is, I believe, the camera you are referring to
Going on the information there, I had a look here
Wiki article regarding the sensor format
Wiki article regarding equivalent focal lengths
On the strength of that, 300 mm on that camera should be roughly equivalent to a 480 mm. lens on a 35 mm. camera. I don't know if that means anything to you, but it is a reasonably serious telephoto focal length. I won't get you a picture of a sparrow in flight that fills the frame, but for a group of ibis a hundred yards away it should be fairly useful.
I love telephoto lenses. 😉
I've got a Pentax digital. I believe the sensor is a fraction larger than yours. I have a zoom that goes out to 300 mm., and find that it covers most normal situations quite well.