"There are five desks."
が introduces it. If you use は you'd be talking about something pertinent to the conversation or already apparent not something that needs introduction. Five desks exist: が.
が also is an action enabler. The desks are existing so use use が to connect them to the verb あります. Just like 日本語が話せます; 日本語 is being introduced and it's doing the verb 話せます (being able to be spoken).
If you put は, as in 私は, then that is being made the topic of the sentence. 私は日本語が話せます: me (topic) japanese (doer) can be spoken (話せ is the potential form).
TLDR: use は to talk about something and が to say it's doing something.
This video says: "As the subject particle が indicates existence." Not that it's the only function.
Basically for 1 to 9, modern Japanese adds small っ to make the readings at least 3 syllables. いつつ is what was spoken in old Japanese, and it got unchanged in modern Japanese.