What is the maximum furnishing I can have in my Genshin Impact Serenitea Pot?

Furniture placement in the Serenitea pot has (at least) two limits:

• number of types: this is clearly indicated in human readable format - it is the "Types 20/200" info you already saw.
• total furniture load: not clearly shown, hidden behind a three color indicator that changes from green->yellow->red. It is the small circle near the word "Load"

The "Types" limit is basically an hoax (like the sky). It is quite hard to reach that unless you try. The Load limit on other hand is your real enemy.

The idea is that every furniture has a "Load Cost". I won't list every cost here for sake of brevity, please refer to this page for a full list.

Each "area" has a separate load limit. External areas can go up to 10k, house rooms can take 4k and the main hall can take 10k (note: some suggest that different platforms have different limits, but that must be an hoax too since a mobile player can also play on pc due to the "cross play" feature...).

Exterior Area Load Limits
All Exterior Areas in each Realm have a load limit of 10,000, while the load indicator will turn orange at 6,000 and red at 9,900. Note that the load of the Main Building will count against the Area in which it is placed.

Interior Room Load Limits The rooms within the Main Buildings, with the exception of the Main Hall and the Corridor, have a load limit of 4,000. The load limits of the Main Hall and the Corridor are 10,000 each. Note that the load amount of Ceiling Lamps will count against the rooms to which they belong.

In your case, just the main building (1440) and the Poll (1800) adds up to 3240 load (on a max of 10k). You also placed two companions (should be 1200 load EACH).... You are already up to 5640/10000 with just these four items alone... Trees by comparison are "small fleas" (having an average load cost of 100-50), so removing those won't do much.

As you can see, sadly the Housing system is severely crippled by the load limit that supposedly exist as a performance protection feature. This in turn mean that you probably will want to avoid big-cost items like animals / companions in already dense areas. Yet, even doing that, it is kinda problematic to juggle load values around and still be able to create really nice stuff. Most of the "Serenitea builds" that you see on Youtube are actually very small areas compared to the available space in the pot. This will only get worse when the road paving furniture that was hinted in a previous Dev interview will be released - want a road? More load!.