I know that most of the vintage road bikes have a rear spacing of 126mm to accomodate 6spd freewheel drivetrains. However, I was reading this thread:

https://www.bikeforums.net/showthread...-an-older-bike

and it seems like I can squeeze in a 9 speed cassette? There are a lot more wheel choices and 700c wheelsets available in my area so I'm asking for future refence.

https://seattle.craigslist.org/oly/bik/3248049275.html

You can cold set a 126mm spaced frame to accommodate more speeds than 7, but I don't know if or how much the size of the frame might limit this, as the dropout face alignment might be more affected on a small frame as the stays are shorter.

Chombi

I do it all the time, but Chombi is right; it often depends on the frame.
I tried it with a Schwinn Aluminum 564 and darn near lost a finger.
I've done it on one Cannondale, and if she gets a rear flat, she'll have to use a cell phone.
An aluminum Centurion was pretty difficult, also.

Most steel frames, especially the larger ones, accommodate the extra width with little more than some extra effort.

The hazards are: misalignment, chain rub on the smaller cogs, knuckle-busting when removing or installing the rear wheel, and sometimes the dish. I've never had the misalignment problem or the dish problem, only had chain rub on an older carbon frame, but have pinched a few fingertips. I'm serious in that you can easily nip off the end of a finger in one of those aluminum frames, or simply get it stuck in there, and you'll not get it out with the remaining unstuck hand.

I squeezed an 8 speed set into a 63cm steel frame (126mm) , not too bad getting the wheel in and out . Robbie is right about clearance for the small cog, as I can't even use it because the chain gets hung up on the frame. Still very much worth the effort .