The Mentor feat allows you to have a cohort after you've successfully guided your apprentice, and yourself a graduate from apprenticeship. If you get the Leadership feat and the Undead Leadership feat do all three stack, as in you'll have 3 Cohorts?
Yes. You'll take all the relevant penalties (Undead Leadership reduces your normal Leadership score, IIRC), but you can take all of them.
Also, remember that the cohort you gain from the Mentor feat doesn't remain 5 levels below you forever. Once he becomes a cohort, he gains XP like one (divide his level by your level, multiply the result by the amount of XP you personally gain, and give him XP equal to the result), so if you decide to stall for a bit by making items, he'll catch up (until he's at most 2 levels below you or 17th). He should actually catch up anyway without stalling yourself, but it's slow going. Better just to crank out some magic items for your crew, and then adventure on. It's not like being one level behind is going to be a problem, considering you bring 2-3 other characters to the table (assuming you pick casters for your cohorts; melee cohorts are usually worth little more than being damage sponges).
The Mentor feat allows you to have a cohort after you've successfully guided your apprentice, and yourself a graduate from apprenticeship. If you get the Leadership feat and the Undead Leadership feat do all three stack, as in you'll have 3 Cohorts?
Yes. You'll take all the relevant penalties (Undead Leadership reduces your normal Leadership score, IIRC), but you can take all of them.
Also, remember that the cohort you gain from the Mentor feat doesn't remain 5 levels below you forever. Once he becomes a cohort, he gains XP like one (divide his level by your level, multiply the result by the amount of XP you personally gain, and give him XP equal to the result), so if you decide to stall for a bit by making items, he'll catch up (until he's at most 2 levels below you or 17th). He should actually catch up anyway without stalling yourself, but it's slow going. Better just to crank out some magic items for your crew, and then adventure on. It's not like being one level behind is going to be a problem, considering you bring 2-3 other characters to the table (assuming you pick casters for your cohorts; melee cohorts are usually worth little more than being damage sponges).
Thank you very much Malkavon. You've ended my stall on continuing my character's feat progression
Which leads to my 2nd question... How many cohorts can you have at one time? For example, say you have just the Leadership feat. Once you've attained a cohort, from that particular feat, can you attract other cohorts or just have one? Or say you just have the Mentor feat. After you've trained an apprentice and s/he graduates and becomes your cohort can you train more and then after they graduate are added to your cohort total or simply replace the one you have?
witch book is the mentor feat in?
It is in the Dungeon Master's Guide II (v3.5 not 4e version)
Which leads to my 2nd question... How many cohorts can you have at one time? For example, say you have just the Leadership feat. Once you've attained a cohort, from that particular feat, can you attract other cohorts or just have one? Or say you just have the Mentor feat. After you've trained an apprentice and s/he graduates and becomes your cohort can you train more and then after they graduate are added to your cohort total or simply replace the one you have?
Honestly, the rules really don't cover it. They don't cover gaining multiple cohorts from any one source, so it really falls into a "the rules don't say I can't" area. You could ask your DM, but I would ere on the side of reasonableness regardless and just assume that if the feat specifically mention gaining multiple cohorts, then you can't do it.