The number of credits available to you appears on the Home screen.
It is displayed in the top right corner of this screen. By clicking on it, you will then directly access the credit store. From this screen you will be able to recharge your account by choosing the credit pack corresponding to your needs.
You can also access this information directly from a race by clicking the "menu" button (the square button with 3 lines in the middle) at the top right of your screen. then by selecting "Credits".
In the window that opens, the number of permanent credits (credits purchased, or credits offered as a start bonus) is displayed, but also the number of credits offered as a start bonus, which can only be used on the ongoing race or leg.