How long to wait between getting new snakes?

I think it's also important to consider how long you've been keeping snakes, I think? I'm still pretty much a newbie, for example, so I'd only get a new snake if all my others were doing fine, had an established feeding routine and no concerning abnormalities (In addition to a stable personal and financial situation etc, like previous posters already mentioned).
In my opinion, especially as a beginner, it's just somewhat better for the successful integration of a new animal if you can watch it really closely and watch out for any new problems that might arise/you may have not dealt with rather than dealing with multiple new animals at a time (which would be the case if you get them in short succession)...
That's why I only got my second snake when my first was close to his second birthday.

But honestly, that's basically just from my POV, maybe other beginners justifyably feel more secure in their husbandry... And I'm pretty sure more experienced keepers can safely care for several new/new-ish animals anyway.