Ubuntu is considered one of the big distros (they have support subscriptions available that are very well received by enterprises) along with RHEL (and the clones) and SLES. If I had to guess I would say that there are more Ubuntu installations then there are SLES - but I have no idea if Ubuntu is more popular than RHEL. This is all based on guesswork though so take it with a grain of salt.
Ubuntu is considered one of the big distros (they have support subscriptions available that are very well received by enterprises) along with RHEL (and the clones) and SLES. If I had to guess I would say that there are more Ubuntu installations then there are SLES - but I have no idea if Ubuntu is more popular than RHEL. This is all based on guesswork though so take it with a grain of salt.