I'm honestly kind of surprised that there's belief that the BJP/NDA will lose a significant portion of seats. Given Modi-Shah's tenacity, what we saw in Surat, public fervor over the Ram temple, my expectation is that at worst they match 2019 or even gain some seats. I might be completely off the mark here but that's my feeling.
I could see them losing some ground in Bihar but making it up in West Bengal.
I do think the BJP will lose some seats overall as keep in mind in 2019 the NDA swept Rajasthan, won 26/28 in Karnataka, won 28/29 in MP, won 39/40 in Bihar so I do think its very possible you could see losses here which cant be made up in West Bengal.
Now the question whether the BJP makes gains or not comes down to whether they can make gains in UP or not as the NDA did lose 9 seats of their 2014 totals there(down from 73 to 64) so if they can get back into the 70s in UP then I think its likely for them to make gains.
At this time though I think we will see the NDA finish somewhere in the low 340s as the total which is higher than 2014 but less than 2019