Learn Spanish (Spain): Los domingos duermo mucho.

In Spanish, you don't use a word like "on" before days of the week. Instead, you just use "the". You use el for a single day (like el domingo for "on Sunday") and los for recurring days. So, los domingos perfectly translates to "on Sundays" or "every Sunday".

I know the verb for "to sleep" is dormir. Why does it change to duermo?

Dormir is a "stem-changing" verb. That means when you conjugate it, the vowel in the middle sometimes changes. For the "I" form (yo), the o changes to ue, giving us duermo. You will see this o to ue change in several other common Spanish verbs too.

Where is the word for "I" (yo) in this sentence?

In Spanish, you can usually drop the subject pronoun (like yo, tú, él) because the verb ending already tells you who is acting. The -o ending in duermo guarantees it means "I sleep", so adding yo isn't necessary unless you want to emphasize it.

Could mucho ever change to muchos to match domingos?