Learn Romanian: Eu exersez mult.

The dictionary form of this verb is a exersa (to practice). In Romanian, many verbs that end in -a add the suffix -ez to the root of the verb in the present tense. For eu (I), that suffix is the only ending you need!

Is it mandatory to include eu in this sentence?

No, you can simply say Exersez mult. In Romanian, the verb ending makes it clear who is doing the action. Including eu just adds a little emphasis, as if you were saying that you practice a lot, as opposed to someone else.

How does the -ez suffix change if I want to say you practice?

The -ez suffix stays, and you just add the standard present tense ending for you (singular), which is -i. So, you practice becomes tu exersezi.

I have seen mult translated as much or many. Why is it a lot here?

Mult is a very flexible word. When used with a noun, it describes quantity (much or many). But here, it is used right after a verb to describe how the action is done. In English, describing an action this way usually translates to a lot.