You need to calculate risk exposure for each of the identified risks then compute a work buffer to cover them. This is part of the Risk Analysis and Management practice.
Since this is a fixed price contract, you need to do an upfront estimation of your work, most likely directly in time units. You also need to sit down and think about what risks may potentially throw off your project of schedule. Then for each risk you think about the probability of it occurring, and how much time this risk will add to your project if it does happen. Multiplying these two gives you a risk exposure that you can then use to build a work buffer.
Now, of course, these probabilities and estimated impact are just that, estimates. Just because there are some steps to follow and some way to compute things, doesn't mean that you will be covered from all risks. You might forget about some risks, you might wrongly estimate the probability of them happening, or their impact. So at the end of the day, it depends also on other factors: what the risk tolerance for your business is, how similar projects behaved in the past, what size the work buffer you end up with is, what it's written in the contract, etc.
EDIT: based on your question in the comment about the fixed end-date, note that there is always a risk that you estimated the work incorrectly. An estimate is an approximation, not a certainty. So you might need to include some buffers for the actual items of work themselves.
For example, it's best to use a 3 point estimate (optimistic, most likely, pessimistic) and then compute the probabilities of completing the project in some duration or another by considering the variance of the activities (see for example this document How to Apply Three-Point Estimating (Program Evaluation and Review Technique - PERT). Once you fix the time constraint with an agreed end-date, you will have to negotiate on the scope or cost constraints, and you always have uncertainty and risk when it comes to estimating scope.