Cap the total global file limit for submissions

### Context

Based on the costs we are willing to incur and the load we are able to handle we need to agree on an upper limit for V1.

Individual files can be up to 10 MB.
The question is how many files or how much combined are we willing to allow per form submission?

### User Stories

> As a **person filling out a form**
> I need to know how many or how much I can append to my submission
> So that I can confidently complete my form

> As a **client administering a form**
> I need to know the limits the system can handle
> So that I can accurately guide my users

> As the **GC Forms team**
> I need to estimate how much we can handle load-wise and cost-wise
> So that I can reduce risks and make informed choices

## Decisions to be made

- [ ] Collective file sizes
---OR---
- [x] Total file number