How many months old can a baby suitable to sit on bumper chair ?

It's more important to see if your child is ready for the chair rather than an age guide (which is usually from 4 to 6 months). What you would want to do is observe is your LOs neck control. If your child is able to hold her head upright and it doesn't flop down...then it's ok to put her on it but for a start, just 5 minutes and slowly increase the time.