Numbers of horses, donkeys and mules
The estimated number of horses, donkeys and mules on all fronts is several millions. Their upkeep, transport and care were a massive challenge, even without reference to any wounding or death. Some 172,815 were sent from India.
Roles played by horses 
Supply horses: 220,187. Supply mules: 219,509. Riding horses: 111,171. Gun horses: 87,557. Cavalry: 75, 342.
The extent of the use of horses is WW1 is set out by the National Army Museum and further illustrated in other resources
https://www.nam.ac.uk/explore/british-army-horses-during-first-world-war; 
https://schoolhistory.co.uk/notes/horses-in-world-war-i/ 
 https://www.bbc.co.uk/newsround/15749027
https://www.thebrooke.org/get-involved/every-horse-remembered/war-horse-facts
And then there were dogs, camels and carrier pigeons
https://giphy.com/gifs/usnationalarchives-archivesgif-wwi-signalcorps-cixJftcJwyARDqgyHV?utm_source=newsletter&utm_medium=email&utm_campaign=signalcorps-march2020
https://rarehistoricalphotos.com/animals-in-world-war-one
And yet others who were mascots
In addition to the working animals, a summary overview from the Imperial War Museum includes mascots and pets  - cats, pigs and even rats
https://www.iwm.org.uk/history/voices-of-the-first-world-war-animals-in-war (20 minute voice recording with transcript)