Hey Kats,
You can put as many bowls on the altar as you like. The bowl and the contents are an "offering" to the theme of your altar. For instance, if you have an altar set up to the four seasons, you may want to put a bowl of flowers on the altar and change them out as they wither. For fall, a bowl of colored leaves, nuts, etc.....things that remind you of fall and so on.
I change the items in my bowl weekly. The offering of the bowl is symbolic of caring for and "feeding" your altar with the energies of the gift.
In Hindu and Buddhist cultures, altars often have food (like rice), sweets, etc that are offered to the diety that particular altar holds dear.
Altars can have whatever you like on them.....many I know, have altars set up to their guardian angel or an angel they resonate with. Candles, incense, sage can all be a part of this altar.
Yes, there are really no true instructions when creating your own altar. You put on the altar what you are led to put on it

Many altars are religious in nature, while others are more personal. There is no one way

Just do what you feel is right. This is your sacred space

I am very drawn to Buddhism and Hinduism myself. On one of my altars, i have a cream colored cloth with gold detail stitching. In the middle is a picture of Sri RamaKrishna, a pic of the Dalai Lama and one of Ghandi. On either side, is a Buddha sitting in the lotus position and a small hand painted Krishna statue. In the front of the altar, I have a hand painted bowl that is currently filled with sage leaves. I also have a rosewood mala that I took with me when i went to see the Dalai Lama in Seattle. I feel it is blessed, so it is there on my altar. I also have a Tibetan bell, tingshas (little cymbals), several crystals, a book (The Teachings of Buddha) in a silk cover, a small brass bowl, a few other books, etc. These items may change from time to time, but this altar pretty much stays as is.
An altar I am currently creating is a gratitude for our animal companions altar

I also have a healing altar in my healing room.
Just remember, this is your personal altar, your sacred space. Create it any way you like!! No rules......just follow your heart

with love and crystal blessings,
Kristi