There are so many galaxies out there, its impossible to accurately know how many there are. The best current estimate is that there are about 100-200 billion galaxies and it has estimated that there could be as many as 500 billion or more. To put that in context, 500 billion is 500,000,000,000. It is estimated that there are 100 billion stars in our galaxy and 200 billion stars in our near neighbour, the Andromeda Galaxy. In other words, there's more galaxies than stars in our galaxy. The real largest galaxy is something that we can never really know.
To put the size in context, the milky way is 100,000 Light Years across, m87 is a million light years across and IC1101 is 6 million light years across. The Galaxy is 1.04 billion light years away, which is a lot further away than our nearest star, Proxima Centauri which is a mere 4 light years.
With such large numbers, its impossible to know. What we do know is that the largest galaxy so far discovered is known as IC1101. It is estimated at having a massive 100 trillion (100,000,000,000,000,000,000) stars which makes our galaxy look tiny. It is located on the borders of Serpens and neighbouring Virgo. Some sites will put it in Virgo and others Serpens. Unlike our galaxy, IC1101 is a dying galaxy. Not many stars are being born unlike our galaxy.
The galaxy was discovered by Sir William Herschel and it was given its name because it was 1101 item in the Index Catalogue of nebulae and star clusters. Even though it is super-large, sorry to use that expression, its not visible like the Andromeda Galaxy to see it unaided. You don't need a space telescope or equivalent to see it, after all Herschel saw it centuries ago.