What can I see using a celestron nexstar 127SLT? I am planning to buy celestron nexstar 127SLT and I want to know what can I see using my telescope.
First of all, the high end of magnification is generally not as important as several other factors. However, considering that no additional information was given in the question beyond "reflecting", I will make some assumptions and only give you a very general estimate.

The book "Fundamental Astronomy: H. Karttunen et al.", page 51, formula (3.5) gives a relationship between the diameter of a telescope's aperture and it's useful magnification.This relation is as follows ($e$ is the resolving capacity of the human eye, $D$ is the aperture diameter, $ω$ is the maximum magnification and $λ$ is the wavelength of visible light, used in the formula for the telescope's resolving capacity:

$$θ ≈ 1.22×λ/D ≈ λ/D,$$

$$ω_{max} = \frac{e}{θ} ≈ \frac{eD}{λ} = \frac{(5.8 × 10^{-4} × D)}{(5.5 × 10^{-7} m)} ≈ \frac{D}{1 mm}.$$

That's a key concept for you, because any additional magnification above that can be thought of as "empty magnification". It exists mathematically, but the image is so blurry that you will not see any additional detail, and probably be quite disappointed in your purchase if you paid a lot of money for that magnification!

Let's try backtracking from 500x. 

If this is the true maximum useful magnification this telescope can achieve, then its aperture diameter would be about 50cm. This aperture allows for $D^2/d^2$ brighter stars to be observed ($d$ is the diameter of the human pupil). When this is calculated and converted to magnitudes, it turns out you should be able to see stars up to magnitude $9+6=15$. 

However this is a very rough estimation and you certainly will not be able to see magnitude 15 stars due to atmospheric extinction, seeing and other factors ... my estimate is you'll see stars about the 12th magnitude on a good night.

If the 500x telescope you are looking at has a much smaller aperture than 50 cm (and probably that's true) then that 500x is not going to be so useful in reality.

Take some time and read further about telescopes. This site has many existing questions and answers on the subject.