Like I've been saying.... It's unlikely that express jet is going to be able to staff ALL of those new planes. Probably some. Their profile says they were planing on hiring 600 pilots in 2019. I wonder if they are on glide slope for that or close. The key is Captains, of course.
So let's look at the airplane numbers.

1. They've been planning on 20+ additional E175s for united for sometime. That's not really news.

2. I'm not sure how many of their current 13 are included in that number but their profile also says they are expecting 25 more... Doesn't mean it's based on a concrete deal but it's something to start from.

3. On June 19th United placed a FIRM order for 20 E175s and optioned 19 more. The deliveries for THIS ORDER are slated to start 2nd quarter of 2020. (aka 1Jan2020). However,....
4. As of June 30th Embraer reported 38 FIRM OUTSTANDING E175s owed to United Airlines.

Soooo

Who can staff them?

Who can upgrade or hire enough CAPTAINS.

Hiring 600 pilots doesn't mean much if 589 of them have zero 121 time....