Misconception
Midi-chlorian are not The Force nor they create it. They are just "indicators" of the innate potentials. They do not determinate the upper cap you can reach, since training in the Force makes you stronger;
as noted in Star Wars official site:
So, despite blood-tests that show your aptitude, learning the Force is still like Yoga, or boxing. You might be born more lithe, or bigger and stronger, but, because we all have midi-chlorians in us, we can all develop the Force to lesser or greater extents — in the world of Star Wars.
and in the Star Wars wiki:
A misconception exists that rather than being indicators, midi-chlorians actually are the Force or create it; there is no canonical basis for this belief.
I think there are continuity errors between the differents canons.
There are at least 3 differents canons you can follow here, and I'm ignoring all the Legends stuff.
- G-Canon - Original and Prequel trilogy, plus George Lucas statements.
- T-Canon - For what we need here, Clone Wars
- Disney's 2014 Reboot - All the movies, TV-series and all novel/comics/VG made AFTER 2014, with some exceptions.
You can found more info on the various canons here.
In G-Canon, there is a statement wich confirm Vader lose strenght with the Force following the injuries he received so it's safe to assume it'd be the same for Maul, but Maul does not appears in G-Canon after Episode I.
In Clone Wars Maul appears near the end of S4. Here we are in T-Canon and there is no statement about the loss of power of Maul, the only consequence of the events of Episode I seems to be his mind shattered.
After 2014, in the current Canon, there are different novels, labelled as canon, wich states Vader grown stronger with the Force after losing half his body.
I am not considering Legends ( or Expanded Universe ) as it contains even more different approaches to the questions that might not be what the op is looking for.
From here, I'll give an anserw following the current Disney's Canon and assuming Midi-Chlorians count determine your power with the Force.
Midi-chlorians count refer to their number per cell.
In Episode I: The Phantom Menace there are some information about the midi-chlorians, in particular:
Thanks to Qui-Gon Jinn we know that Anakin had a count of
Over 20,000. Even Master Yoda doesn't have a midi-chlorian count that high.
And when he speak later with Master Windu he says
His cells have the highest concentration of midi-chlorians I've ever seen in a life form. It's possible he was conceived by the midi-chlorians.
In The Making of Star Wars: The Definitive Story Behind the Original Film Lucas says
It is said that certain creatures are born with a higher awareness of
the Force than humans. Their brains are different; they have more
midi-chlorians in their cells.
The website theforce.net quote an explanation of Lucas where he states
Midi-chlorians are a loose depiction of mitochondria
That said, losing part of your body doesn't lower your midi-chlorians count, because this refer to the number of midi-chlorians in a single cell, not in your whole body.
Regarding Darth Vader, in both Tarkin and Lords of the Sith is explained that his connection to the Force were strengthened after his duel with Obi-Wan in Episode III, while his body were severely crippled, and further states
Though his injuries had physically crippled him, Vader believed that the suffering that he had experienced had both perfected his spirit and freed him from the needs of the flesh, allowing him to concentrate solely on his relationship to the Force.
Back on the question:
- Anakin/Vader doesn't have his potential lowered due to losing midi-chlorians when he lose his limbs.
- There is nothing that suggest Maul lose power after being cut in half. What it's know, is that his mind shatters and only after years and the help of Mother Talzin he was able to recover from his state.