Cracks are not all equal
A clean break right across a tile needs attention. A small surface chip at the bottom edge is different and may be mostly cosmetic. The trouble comes when a crack lets water through the body of the tile or allows one section to move.
How tiles get damaged
Storm debris, ladders, foot traffic and frost can all damage tiles. So can poor fitting. If a tile is forced into position or nailed too tightly it may crack later as the roof moves with temperature changes.
One replacement can be enough
Where the rest of the roof is sound, replacing the damaged tile is usually the sensible repair. I would still ask why it broke. If several tiles in the same area are fractured, especially around a chimney or roof junction, there may be another cause behind it.
Do not rely on surface sealants
Paint-on fixes can look convincing from the scaffold for a few months. They are rarely my first choice on a cracked tile. A sound replacement is usually cleaner and easier to judge later.