Acharya is used in two senses. Anyone who teaches another person will be an acharya for that person. The master's degree in Sanskrit is known as the 'Acharya' examination (my grandpa was one). However, Acharya is also used to honorify the greatest philosophers (or great scientists and astronomers) in Hinduism - Sankaracharya, Ramanujacharya, Nimbarkacharya, Vallabhacharya, Madhvacharya, and Chaitanya, who are Acharya even for people who may not follow their philosophy.
The first Sankaracharya established four seminaries in four corners of India (Dwarika, Jagannatha Puri, Jyotirmatha and Shringeri). In time, a fifth was established in Kanchipuram. The heads of these seminaries are now known as Sankaracharyas.
Mahamandaleshwar is the chief head of a religious order, accepted in time of Sankaracharya (he established ten) or later (now there are thirteen, three belonging to Sikhs). If the order has branches in other places, their Chiefs would be known as Mahant. There are fake seminaries and fake Mahamandaleshwaras.
The Sankaracharyas and Mahamandaleshwaras may also be termed as Jagadgurus (Jagat+guru = Teacher for the whole world).
Maharshi (Maha+rishi = Great Sage) and Paramahansa are not official titles. These are accepted by public opinion, like Gandhi being called Mahatma. One who is a Maharshi or Paramahansa for one, may not be for another. There are fake Maharshis and Paramahansas. I do not accept Yogananda though he was popular in West.
"Of those who do belong to a group, the thirteen active akharas have been,
- 7 Shaiva akharas: Mahanirvani, Atal, Niranjani, Anand, Juna, Avahan, and Agni
- 3 Vaishnava akharas: Nirvani, Digambar, and Nirmohi
- 3 Sikh akharas: Bara Panchayati Udasins, Chota Panchayati Udasins, and Nirmal
The ten Shaiva and Vaishnava akharas are also known as the Dasanamis, and they believe that Adi Shankara founded them and one of their traditional duties is
dharma-raksha (protection of faith)."
Kumbh Mela - Wikipedia