Shopping mall is not a bad place to approach. I find it a little challenging, but nothing that can't be handled. Anyway, you can always mix it up by trying specific shops: Bookshop, department store are my favourites...
Getting numbers is not really the primary goal, it's getting dates... be sure to always ask for the date first, and if possible discuss a time, date, place at least in general terms... the number is only for if she has to cancel or if the details couldn't be determined ahead of time.
Another thing to consider about numbers is the quality. With a little practice you'll know which numbers are hot, by factors such as her demeanor and body language, how she saves it in her phone, how invested and engaged she was in the interaction, how you felt you performed and she responded, etc...
The thing that will get you dates is basically your conversation. So you've opened... preferably direct... now you might want to sprinkle in a few lines of banter...
You: hi there... I could not help but notice your skirt, it goes great with your outfit... you look fantastic and I wanted to meet you. I'm partytillyoung.
Her: I'm HB8
You: We're actually almost twins, got that denim/black/white look going... how about we swap jackets, that would look cool
Her: haha you'll have to try and wriggle into it
You: I'm a size 8! your honour!
Her: haha
You don't need any banter at all, or you can sprinkle it in later (or use sexual humour instead)... but if you do a few lines is sufficient as above, then try a cold read
You: you've been shopping at my favourite shop, can't get enough of uniqlo... is that all for today, u were heading home now?
Her: no I was gonna meet my mum for coffee, she's in another part of the centre
You: (thinks: shit, I had better grab a number before the mum turns up) oh cool, is the shopping with your mum a regular thing?
Her: oh not really it's just when we feel like it... I didn't have work today and hadn't seen her in a bit, so we met down here
You: (checks logistics) oh right so you live somewhere close by? like a share house or something?
Her: no well I'm coming from (suburb), I have one roommate but it's not really a share house
Normally would ask a bit about the work as well and her routine but since mum is on the way, let's pull the trigger
You: cool... I live near there too, maybe we could meet up sometime, since you obviously don't have much time today?
Her: ookay?
You: great I am enjoying talking to you, if you are free say Thursday or Friday that might be okay?
Her: okay well I'll let u know, I haven't got my schedule yet
You: awesome so lets just exchange numbers and take it from there then... pass me your phone
You: (puts your number in) okay if I call myself?
Her: sure (you call yourself)
Now that that's sorted we can't let the vibe die straightaway... have her put her phone back in her bag so mum won't see it... and continue thus
You: so you mentioned you have the day off work today?
Her: yeah I don't work Tuesdays... I'm a dental nurse
You: oh awesome! you must meet a lot of interesting people
Her: yeah some of our patients are hilarious, I don't think they mean to be, but sometimes its a shame to put stuff in their mouths cos it stops them talking
You: haha, and you get on well with your boss?
Her: yeah she's cool, although I don't know her that well, I only work 2-3 days a week and there's another nurse who does the other days
(her mum comes up)
You: oh hi you must be HB8's mum... I'm partytillyoung, did u get your shopping done?
Mum: yeah nice to meet you
You: okay well I'm sure you've got lots to catch up on, I'll leave you to it (smile, take leave)
That's pretty much what a successful interaction looks like, notice that when we asked a question we either phrased it as a statement, using only a very slight amount of tonality to prompt her to answer, and/or we provided a suggested answer to make it easier for her (this is what a cold read really amounts to).
Ray