yad yad a~Ngam vihiyeta tatsaMkhyAdviguNo japaH|
kuryAd dvitrichatuHpa~nchasaMkhyaM vA sAdhakaH priye||
kurvita chA~Ngasiddhyartham tadashaktau sa bhaktitaH|
tachcheda~Ngam vihiyeta mantri neShTam avApnuyAt||
-kulArNavatantra,pa~nchadasha ullAsa (15th chapter,Kularnava tantra)
यद् यद् अङ्गम् विहियेत तत्संख्याद्विगुणो जपः।
कुर्याद् द्वित्रिचतुःपञ्चसंख्यं वा साधकः प्रिये॥
कुर्वित चाङ्गसिद्ध्यर्थम् तदशक्तौ स भक्तितः।
तच्चेदङ्गम् विहियेत मन्त्रि नेष्टम् अवाप्नुयात्॥
If one limb of a five limbed purashcharaNa is lacking,one does japa dviguNa(twice),triguNa(thrice),chaturguNA or 5 times to make up for it. [Apparently according to Buhnemann,a text called PA;perhaps is it the purashcharyArNava? states that the number of times one has to repeate this substitute japa depends on the varNa of the person doing the purashcharaNa,like brAhmaNas doing 2x,and the others in increasing orders:I’m not able to check for myself the original text of the purashcaryArNava].
Anyway,these are the range of rules the tantra has stated,what suits the individual sAdhaka is best learned from his guru who can clarify(and is the ultimate authority).
So,if one had the homa saMkhyA 10,000:He would have to do at least 20,000 japa to make up for the missed limb(a~Nga) of the purashcharaNa.
Practically,the author has seen mostly people recommending 2x or 4x to make up for the a~Ngas that could not be done(like homa,tarpana,etc).
Another note from muNDamAlA tantra(5h paTala):
nyunAdhikam na japtavyAmAsamAptaM sadA japet|
rAtrisheshe japenna cha|
yastha yAvAn japaH proktastaddashAMshAmanukramAt|
tattaddravyairjapasyAnte homaM kuryAd dine dine||
homasya cha dashAMshena tarpaNaM proktameva cha|
tarpaNasya dashAMshena shiromArjanamiShyate||
taddashAMshena viprendrAn kurvIta kulakanyakAH|
homAdyAshakto deveshi!kuryAchya dviguNaM japam|
yadi pUjAdyAshaktaH syAd dravyAlAbhena sundarI|
kevalaM japamAtreNa purashcharyA vidhIyate||
Translation of the above relevant points that I cited:One should not do more or less japa every day till the very end. One should not do japa at the end of rAtri. One should do successive dashAMshas of homa,tarpaNa,mArjana,and brAhmaNabhojana. If one is incapable of homa,etc,he should do 2x the japa. If he is incapable of pUjA etc a~Ngas due to lack of dravyas,by japa alone he can complete the whole purashcharaNa.
Swami Sacchidananda Saraswati in his Purascharanapradipa states: অশক্তকল্পে:কেবল জপে জপেই যে কোন সাধক পুরশ্চরণ হইতে পারে। এমন কি ব্রাহ্মণভোজন অঙ্গের অন্যকল্পেও জপদ্বারাই তাহা সিদ্ধ হইতে পারিবে। যথা:-(and then quotes the last shloka I had cited here).
Personal note(I am no AchArya):pratyakSha brAhmaNabhojana is still the best option even if one has substituted all the other a~Ngas with japa for the reason it falls in the same category as kumAribhojana:Both of these remove all the doshas that might have been committed in the process of the mantrasAdhanA. And of course,the brAhmaNas should be vedaj~na or should be doing upAsanA of the same mantra as the sAdhaka is doing/be dIkShita in that same mantra.