Some guys bring their own because they have a preferred brand or they're worried the girl will run out or "forget" to get them, but the norm is for her to have them.

Or you can use the house supplied ones. Most providers that I engage allow me to use my own if I want, most of the careful ones inspect the package to make sure there was no tampering. I have had cases where I have given ladies excess condoms after our business was done.

Unless you have have a special request - like an allergy to latex or the lubricants often used on condoms, or you need a special size, etc.  If you have those kinds of considerations than by all means bring your own, just make sure the box is sealed.  Providers can get nervous when presented with condoms.  There are stories of hobbists tampering with condoms and you don't want to spook someone on the first visit.  Have fun out there!

However there are scams where no second pop (time permitting) because only 1 condom.  Or the variations of "take the money & run" scams...  (condoms in the car, drive me to the drug store, etc).

I have found it's convenient to have a sealed 3 pack of my prefered brand...  I drop on the bedside table before excuse myself to the bathroom (she can inspect for tampering & expire date).   I leave any remainder.  
I respect that a lady has the right to refuse to use condoms I supply, but it's never happened to me.
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don't show them unless you have to. I have read about providers who accidentally ran out or were just irresponsible.

I can also see a prime opportunity for a "cash and dash". You know, she says she'll go to the corner drug store real quick and disappear with your money.

Do remember that condoms are the smoking gun of hobby involvement.  If you are married or hooked to a SO, don't ever keep condoms in house/car/briefcase. If you really want to supply them (I assume that's the lady's responsibility), buy on the way and leave the remaining with her.

who will not have, or pretend to not have, condoms to avoid some activities. Having your own (boxed and sealed) can help.