The problem is not with the "number closing". Although there are certain techniques developed by PUAs (like David D's "give me your email" and then asking her for her number as well, I don't think they are particularly important. Asking for the number should come naturally when you part.
If you fail to get numbers, the odds are that your overall game is deficient. But don't worry about that, we all have been there. If we were a bunch of naturals, we wouldn't be posting on this forum
Just keep working on your game. Keep it light and funny and don't act like you want something from the girls. It will gradually come to you and women will start giving you their numbers even without you asking
Good luck and have fun in field!