You know it's called a "FALSE time constraint" right?
Basically you're not building any comfort with them, so the numbers will all be flakey because they haven't invested emotionally in you at all. You're just another name and number in their contacts list. The number itself doesn't mean anything, it's just a way to ensure a day2. Get a number AFTER you've spent at least 10 minutes and you've found at least some commonality or have a few jokes with each other. This is where things like nicknames (smiley, bratty, dorky), negging her on specific traits, future projection (making future plans to do something, e.g. "omg I went to the COOLEST place last weekend with some friends, it's this wine bar that only serves wine, beer, cheese and chocolate and the menu has all of these amazing pairings, have you heard of it? No? Oh well we gotta go sometime!") and just finding simple commonalities come in handy. For example since I have a massive collection of music, I can usually find stuff that any girl, regardless of her tastes, would like. So a simple music conversation will lead to "have you heard of X band?" Then I say "oh well I gotta play you this song from their album sometime." That's useful for future projection, but also to counteract her ASD when it's time to take her home, because we're only swinging by my place quickly so I can play her that one song, and then I gotta get up early
Once you get comfort, then get the number (and stay another few minutes so it doesn't feel like you're just there to get the digits and 5uck off) and a text within 24 hours with a playful and fun vibe to it, where you use her nickname you gave her, tease her about her terrible music taste, and mention the cool cafe you told her about, will be a lot more warmly received than then the "hey it's X from bar Y remember me let's get a drink I'm gay" text you usually send.
