Generally, the number of languages spoken within the household is the number of languages a baby gets to hear, maybe with one or two extra. The best way for a child to learn to understand a new language is for them to hear people speaking it fluently. They will pick up the sounds and the natural accent.
The ability to hear different phonetic pronunciations is sharpest before the age of three. Two and three year olds love to mimic what they hear, and soon they will begin to understand the meanings of short words and phrases.
It is quite common in Indian households for the babies to be exposed to a number of languages within the family, and the people around them. The child interacts with the maid, the driver and other people who may not speak the same dialect or language. The only way a child can truly master a second language is if they are frequently around someone who speaks it fluently. Slowly but steadily they will start conversing in the languages they hear. Children are fast learners, so they are able to pick up new languages.
In case the parents belong to different linguistic communities, choosing a primary language can initially be slightly challenging. Following the patriarchal system, the mother tongue of the father becomes the primary language for the child. But there is no hard and fast rule on this. Teaching many languages especially the mother tongue is important. If they do not get to hear the mother tongue, they will never learn.
You can speak to the child in any one language primarily and also introduce words and phrases from the second language gradually. The child will learn to converse in both languages, and gain fluency over time by talking to relatives on both sides in their respective languages.
Learning a second language improves memory and analytical abilities. Being multilingual should be seen as an investment that pays off for a lifetime.
Unless the child is confused, they can learn as many languages as they want. It is quite natural for a child to confuse the word order or use words from both languages in the same sentence. These are minor mix-ups and they will soon learn to separate the languages.