I find those little bottles of sealer with the brush/roller on the cap
tedious. Grout sealer will not harm/discolor your tiles if they are
glazed. I just pour it over the tiles and spread it around with a sponge,
wiping up any residue on the tiles with an old rag. You can also use a
paint roller, foam paint brush, spray bottle, what ever is easiest for you
to apply. Just wipe up left over droplets off the glazed portion of the
tile with a rag.

Mark

What is the best way to seal the grout lines on a glazed ceramic tile
floor? There are many grout lines since the floor consists of many
small tiles: 1x1, 1x2 and 2x2 mosaic pattern. Can I just brush the
silicon over everything and then wipe the tiles clean after it dries?
It would be very time consuming to apply the silicon just to the
grout lines.

I'd get one of the bottles with a brush in the cap intended for
sealing grout. It won't take long to paint all the lines, even
with 1" tiles.

I doubt the sealer would hurt the glazed tiles though. Clean up
with some water and a blue/green Scotch-Brite pad.