In the feud between Lancelot and Arthur after Lancelot's betrayal, which knights sided with Lancelot and which knights sided with Arthur?

You can find the answer to this question in Books XX and XXI of Malory's Le Morte d'Arthur, assuming that you are referring to the feud that ensues after Lancelot's infidelity with Guinevere is exposed to Arthur.
Sadly for them, several of the knights who are slain by Lancelot actually expressed their concerns to Arthur about the plan, not wishing to make war upon Lancelot. Arthur asks Sir Gawaine and his brothers Sir Gaheris and Sir Gareth to "bring my queen to the fire." Gawain objects to this. However, intelligence means that Lancelot is able to launch an attack, knowing what Arthur plans to do, and "as Sir Launcelot thrang here and there, it mishapped him to slay Gaheris and Sir Gareth." A number of other knights stood against Lancelot and were killed, including: Sir Belliance le Orgulous, Sir Segwarides, Sir Griflet, Sir Brandiles, Sir Aglovale, Sir Tor; Sir Gauter, Sir Gillimer, Sir Reynolds' three brethren; Sir Damas, Sir Priamus, Sir Kay the Stranger, Sir Driant, Sir Lambegus, Sir Herminde; Sir Pertilope, Sir Perimones, two brethren who were called the Green Knight and the Red Knight.
It is the slaying of his brothers that has Sir Gawain prevailing upon King Arthur to make war upon Lancelot for what he has done. Arthur agrees and sends writs and summons throughout his kingdom to assemble what is to become "a huge host" led by Arthur and Gawain. Lancelot, we are told, fears the sheer size of Arthur's supporting army but is not without support of his own, some driven by love of him, and some loyal to the Queen. Supporters of Lancelot's included "Sir Bors de Ganis, Sir Ector de Maris, and Sir Lionel... Sir Palomides, Sir Safere's brother, and Sir Lavaine, with many more of their blood."