There are 81 (3^4) possible states of corners twisting in Megaminx.
There are 16 (2^4) possible states of edges flipping in Megaminx.
Thus, there are 1296 states of the orientation of last layer (OLL) in total (including OLL skip).
There are 60 (5!/2) possible states of corners permutation in Megaminx.
There are 60 (5!/2) possible states of edges permutation in Megaminx.
Thus, there are 3600 states of the permutation of last layer (PLL) in total (including PLL skip).
Most of the LL cases can have up to 25 variations. Except the following 10 PLL cases where each of them can have only 5 variations:
1) O (PLL skip)
2) H1+
3) H1-
4) H2+
5) H2-
6) N2+
7) N2-
8) X1
9) X7+
10) X7-
Thus, the 3600 states of the PLL cases can be reduced to 151 algorithms and 1 PLL skip because:
(142 * 25) + (10 * 5) = 3600.
1296 * 3600 = 4665600 is the number of all possible 1LLL states.
There are 4665600 - 3600 = 4662000 OLL cases because the LL skip and the PLL cases have been excluded.
Then, we can have 4662000 cases / 25 variations = 186480 distinct full non-PLL 1LLL cases.
All of these 186480 cases are reduced to 259 full OLL cases in 2LLL because we consider only the states of the grey color pieces.
186480 / 259 = 720 means that there can be the algorithms like 01A_001, 01A_002, and so on until 53E-_720.
Finally, we have 186480 non-PLL 1LLL cases, 151 PLL cases, and 1 LL skip.
The total number of 1LLL algorithms for Megaminx is supposed to be 186631 algorithms (or 186632 cases with 1LLL skip included).