Kevin,
Creating a Customer Attributes data source does not count against any limits. You can always add new data sources, so long as they have a unique alias ID. The only limits or "slots" as you called them are around subscriptions. You can find more about the subscription limits here. I've posted them below for convenience.
When configuring subscriptions and selecting attributes, the following limits apply depending on the solutions you own:
- Analytics Standard: 3 total
- Analytics Premium: 200 per report suite
- Target Standard: 5
- Target Premium: 200
Thanks,
Jantzen