Can you please assist me with a formula to calculate number for months say from 01/05/1999 to 31/08/2025

Depends how accurate you want it.

You can just take one date and subtract the other and divide by 30.5 

... assumes stop-date >= start-date

if year(stop-date) = year(start-date)
then Months = month(stop-date) minus month(start-date) plus 1

else Months =
12 minus month (start-date) plus 1 ... beginning stub
(year(end-date) minus year(start-date)) times 12 ... great middle
month (stop-date) ... ending stub
end if

Per example ...

... 12 -5 +1 = 8
... plus (2025 - 1999) *12 = 26 * 12 = 312
... plus 8
... total is 328 months

not bad