You can select as many books as you wish, then apply the search/replace.
Changing the title can be quite time consuming in calibre. Changing one title could take approximately a second, so the number you change at a time will depend on your patience. Changing large numbers of books could make it harder to eyeball the results.
I would probably do a screen at a time, checking the results. I would save the search/replace to make it easier to repeat it on the next screen. I would choose this approach because I am a bit paranoid about using "fire and forget" with this sort of operation, a trait you might not share.
