Ravana had six brothers and two sisters: I know about him but how many sons of Ravana are Except Indrajit?
Ravana had 7 (maybe 6) sons including Indrajit. However I won't be including Indrajit in my answer.
Valmiki Ramayana Yuddha Kanda Sarga 71
tasyaasiidviiryavaanputro raavaNapratimo raNe |
vRiddhasevii shrutadharaH sarvaastraviduShaam varaH || 6-71-28
To him, there is a valiant son, equal to Ravana in combat, who serves elders, holding that which is heard from them and skilled in the use of weaponry.”
ashvapRiShThe rathe naage khaDge dhanuShi karShaNe |
bhede saantve cha daane cha naye mantre cha saMmataH || 6-71-29
He is respected for his counsel, for his rides on the backs of horses and elephants, for his drawing out of the sword and the bow, for his strategy winning over to his side by sowing dissension, negotiation and bribery and for his steering capacity of an army.
yasya baahum samaashritya laNkaa bhavati nirbhayaa |
tanayam dhaanyamaalinyaa atikaayamimam viduH || 6-71-30
He is Atikaya, the son of Dhanyamali. Lanka is feeling fearless, by resting on his arm.
Sundara Kanda Sarga 47
senaa patiin panca sa tu pramaapitaan |
hanuumataa saanucaraan savaahanaan |
samiikShya raajaa samara uddhata unmukham |
kumaaram akSham prasamaikShata akShatam || 5-47-1
Hearing and sustaining that the five army-generals along with their followers and vehicles were disposed off by Hanuma, Ravana looked at Aksha, his son, violent in combat and who was inclined to fighting as well as sitting in front of him.
Yuddha Kanda Sarga 69
shrutvaa trishiraso vaakyam deva antaka nara antakau |
atikaayasH ca tejasvii babhuuvur yuddha harShitaaH || 6-69-9
Hearing the words of Trishira; Devantaka, Narantaka and the energetic Atikaya were rejoiced of war.
tato aham aham iti evam garjanto nairR^ita R^iShabhaaH |
raavaNasya sutaa viiraaH shakra tulya paraakramaaH || 6-69-10
Then, the brave Ravana's sons, whose prowess was equal to Indra and the foremost of demons, roared asserting their superiority saying "I will lead, I will lead!
Regarding Prahasta is it unclear whether he is Ravana's son, or just an army general.