I'm assuming you are talking about the hitbursts the appear showing how many points you got for hitting a note? For example 300, 200, etc. Or are you talking about the combo numbers themselves? Combo numbers can be hidden with shift+tab, but the hitbursts cannot.
Both of these are always vertically centered on the stage, but you can change the horizontal height through whatever skin you are using. In osu!'s options menu search for and select "Open current skin folder." Look for a file named "Skin.ini" and edit it with notepad. There should be some mania sections that look like this:
[Mania]
Keys: #
...
ScorePosition: #
ComboPosition: #
...
Changing the ComboPosition value will change where the combo numbers appear horizontally on screen and changing the ScorePosition value will change where the hitbursts appear. This has to be changed for each mania section if you want it to apply to every number of keys. Or you can set different values for different keys.
If you are using the default skin, create a skin.ini file and add the [Mania] sections in yourself.
For example, your entire skin.ini file can look like this:
[Mania]
Keys: 4
ComboPosition: 175
ScorePosition: 100
[Mania]
Keys: 5
ComboPosition: 1
ScorePosition: 0
When you are done editing the file, save it, and reload your skin in game with the shortcut "ctrl+alt+shift+s"
There are other settings in the skin.ini file you can play around with and there is a reference
here.