Usually a 'P21W' 21w Single filament bulb if a separate brake light lamp, or a 'P21/5W' 21/5W Twin filament bulb if a combined tail light/brake light lamp.
If a twin filament bulb, careful how you fit, they have offset locating pins to ensure the bulb uses correct filament for each function. Many bulb holders are poor quality and a loose fit, and it's possible to fit incorrectly with not too much excessive force! Check after fitting that the illumination levels are correct for function!
The existing bulb should have it's ID stamped into the cap, or occasionally etched into the glass.
Don't be tempted to substitute a LED replacement bulb instead of traditional filament bulb if you wish to remain road legal, replacement LED bulbs are NOT permitted for road use, as not certified and 'E' marked!
Normally you undo the screws and gently pull off the outer cover revealing the bulbs. Alternatively the whole un8t comes off and you access the bulbs from the rear.
------------- DS-There's more to life than football!!!