Mimi nina marafiki wachache.

In nina, the ni- already marks the subject I. So Mimi is optional and mainly adds emphasis/contrast, like As for me / Me, I have...

• Neutral: Nina marafiki wachache.
• Emphatic/contrastive: Mimi nina marafiki wachache (lakini yeye ana wengi). (I have a few friends, but he/she has many.)

Nina is a compact form with multiple pieces:

• ni- = subject marker I
• -na- = present tense marker
• (implicit verb base -na meaning have/with)
  So ni-na-... gives I (present) have.... You’ll also see:
• una = you (sg) have
• ana = he/she has
• tuna = we have
• mna = you (pl) have
• wana = they have

Swahili often expresses having using a structure that’s literally closer to be with. The key idea is -na = with/and, and with subject + tense markers it functions like have:

• Nina X ≈ I have X (literally I am with X)
  This is why it behaves a bit differently from an English verb in some contexts.

Rafiki is singular (a friend). Marafiki is plural (friends).
This is one of the noun classes where the plural uses ma-. So:

• rafiki = a friend
• marafiki = friends

Wachache (few) agrees with a plural noun in a way that uses the wa- form for this adjective. Many quantity adjectives (like -chache) have agreement forms that don’t always match what learners expect from the noun’s ma- prefix.
So you get:

• marafiki wachache = few friends
  Even though the noun begins with ma-, the adjective form used here is wa-.

Yes. The common order is:

• noun + adjective → marafiki wachache
  This is the normal pattern for descriptive adjectives and quantity adjectives.

To negate I have, you use the negative subject form plus -na:

• Sina marafiki wachache. = I don’t have a few friends / I don’t have many friends (context decides the nuance)
  Here si- is the negative for I, and the present negative drops the -na- tense marker you saw in nina.

A common way is to add tu (only/just):

• Mimi nina marafiki wachache tu.
  Or without emphasis:
• Nina marafiki wachache tu.