Partial answer:

>What is a "space launch delta", 

Essentially what the 30th (and 45th) Space Wings used to be. 

>how many of them are there, and 

Two, under Space Systems Command, US Space Force.

>what fraction of them are assigned to Space Force?

All of them? It is what the 30th and 45th Space Wings used to be, and are still based.


It is actually a relatively new structuring of the US Space Force

**Space Force Looks to Operationalize New Structure, Build Culture in 2022**

https://www.airforcemag.com/space-force-looks-to-operationalize-new-structure-build-culture-in-2022/

>"We have gotten rid of the numbered air force echelon.." 

>“So we’ve taken the base operating support, all that combat service support functions, and we’ve taken those and put those into a garrison..."

>".. and now we have the delta, which is singularly focused on the operational mission that we’ve been assigned.”


>The **delta**, *not the squadron*, is the “presented force” for the Space Force. And compared to the size of Air Force wings and groups, deltas are small, at least when it comes to personnel.

>“...when I was an ops group commander, I started with about 2,000 people, if you add in all the contractors, all the civilians. … Now we’re sitting about 400, **450 total** when we add everything together,”

*Space Systems Command:*

Established at the time when intra-service rivalries had resulted in parallel developments of operational ballistic missiles which could also serve as satellite boosters, and a reconnaissance satellite could provide strategic intelligence on Soviet capabilities . With the other services, the United States Air Force pursued missile and satellite development by establishing the *Air Research and Development Command*'s **Western Development Division** on 1 July 1954, making Space Systems Command the *oldest* Department of Defense space organization.

https://www.spoc.spaceforce.mil/About-Us/About-Space-Operations-Command

>On August 13, 2021, Space Launch Delta 30 at Vandenberg Space Force Base, California and Space Launch Delta 45 at Patrick Space Force Base, California, were realigned from Space Operations Command to Space Systems Command.

https://www.airforcetimes.com/news/your-air-force/2021/05/14/introducing-vandenberg-space-force-base-and-space-launch-delta-30/

**Introducing: Vandenberg Space Force Base and Space Launch Delta 30**

>Vandenberg Air Force Base will become Vandenberg Space Force Base Friday afternoon — the third installation to adopt the new service’s moniker so far.

“For 63 years, this has been an Air Force base,” 30th Space Wing boss Col. Anthony Mastalir told Air Force Times Wednesday. “The moment is not lost on the people here that, when we plant the Space Force flag, so to speak, and we change that name, we’re going to begin a new chapter.”

Vandenberg’s host wing, the 30th Space Wing, is similarly changing its name to Space Launch Delta 30. The shift is in line with other space-focused wings that are part of the Space Force, which is trying to create an identity separate from the Air Force.

Its sister installations in Florida, Patrick Space Force Base and Cape Canaveral Space Force Station, ditched their Air Force titles in December. The 45th Space Wing there also recently changed its title to Space Launch Delta 45.

While the ceremony marks the launch delta’s transfer into the Space Force, it does not change Vandenberg’s status as an Air Force-owned base.

https://www.vandenberg.spaceforce.mil/About-Us/History/

>Space Launch Delta 30 provides host installation services for Vandenberg Space Force Base (SFB).  

>Personnel assigned to Space Launch Delta 30 at Vandenberg SFB manage Department of Defense space and missile testing, range operations, and provides the installation infrastructure necessary to place satellites into polar orbit using both expendable and reusable space lift boosters. 

>Personnel assigned to Space Launch Delta 30 support Minuteman III Intercontinental Ballistic Missile Force Development Evaluation programs. Additionally, Space Launch Delta 30 provides safe launch and range capabilities as the only military installation on the West Coast of the continental United States to fulfill commercial and government customer requirements.

>Since 1964, the host installation, range, and launch missions at Vandenberg SFB have been supported by personnel assigned to Space Launch Delta 30, and its predecessor units. 

>Space Launch Delta 30 manages all launch and range missions on the Western range that are conducted from Vandenberg, including launch activities for the Space Force, Air Force, Department of Defense, NASA, United Launch Alliance, SpaceX, Firefly Aerospace, and other commercial space partner organizations. 

The emblem of SLD30 retains some of the elements from its previous Air Force role:  The blue alludes to the sky, while the Yellow refers to the sun and the excellence required of its personnel (For SLD30, the Yellow is now Gold, representing Space Systems Command). The two launch vehicles emanating from behind the globe represent the 30th Space Wing's intercontinental ballistic missile test and space launch missions (although now referred to as 'Test' and 'Launch' for SLD30). The remaining red, white, and blue elements represent the national colors of the United States.