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).