I did a somewhat cold approach on a girl that used to attend the same fitness class as me at my gym. This was about 6 months back.

First Approach: (2 set)
I probably made the mistake of ignoring her friend, showing a little too much positive body language, scattered/nervous eye contact but had some great banter with my target. Both of us were giggling/laughing and I teased her a little bit when she referred to one of the guys in our fitness class as "the bald guy." The whole thing happened pretty fast, they both seemed really nervous and were not sure how long I was going to stay talking to them. We said our goodbyes and before I knew it they both quickly retreated into the zumba room.

2nd Encounter: (2 set)
Next time I saw her, she completely avoided my gaze. I figured she was uninterested so I kept to myself.

3rd Encounter: (3 set)
On my way out of the gym, she was at the water fountain and we both made eye contact (she broke it first pretty quickly), I smiled and waved goodbye at her and her group.

4th Encounter: (2 set)
On my way out again, she spotted me, so I smiled and waved goodbye. She then started to approach me and intercept the gym exit. I'm not entirely sure if they were finished with a particular exercise and were moving to the next routine, or if she ditched her friend. In any case, she approached alone. She immediately said "Hi, how are you doing?" on her way over to me. We bantered a little bit about our Halloween costumes. I did a little bit of body rocking. She mentioned something about going to a zombie crawl the other week, I replied with something like "oh that's cool... did any of the zombies try to eat you?" with a cocky smile. She did a double take at me and giggled a little.

I'm thinking for the next encounter, should I go just for the number or number + date? Or just hold off on all this and try to build some rapport first?

She, like all girls, go on cycles when they're not into chilling with new dudes and when they are. Encounter 4 was a perfect example of the latter. Speaking only for myself if it were me in this position I would interpret her approach as a sign that she wanted to bone. Next time you're in a nice convo with her schedule a meeting outside the gym - get a juice or a drink or something. Then take the # after she agrees to meet.

Well shit, how long is this cycle gonna last?

Until she takes it doggystyle from the hot dude she meets at a halloween party tonight only kidding.

Assume attraction and assume she is into it and be smooth with it all.