Is it nessary to tribute more than 1 monster for ritual summon?

For example,
Black Luster Ritual

This card is used to ritual summon "Black Luster Soldier". You must also offer monsters whose level stars are equal to 8 or more as a tribute from the field or your hand.

(I just started using ritual cards)

Nothing in the game is "Neccesary"; thing is, the text reads "Monsters" because you will either usually have more than 1 Monster to offer for a Ritual that big, or to point out that you CAN offer more than one monster and not be obligated to offer 1 with the equal Level or more.

So i could use 1 monster tribute for ritual summoning because i would like use koitsu which could be special summoned easily by mother grizzly for ritual summon

You can use as many monsters as you like for a Ritual Summon.

You can use as many monsters as you wish for the Ritual Summon as long as the Level's are equal to or greater than the amount required.

However, you CANNOT tribute "unnecessary" monsters. Like if you tribute 2 Level 4's for Black Luster Ritual, you cannot choose to Tribute a Level 1+ monster since you would have already tributed enough monsters, making this one unnecessary.

Also, there are some Ritual's that require an EXACT number of Level's like End of the World, Contract with the Abyss and Earth Chant.

This means you can only tribute as many as you want as long as their combined level's equal exactly the level of the Ritual Monster you're summoning.