Is it more advantageous to summon on a campaign when more Servants are offered?
So a banner can offer a different range of Servants; sometimes just a couple of them and sometimes all of them at once. Is it better to summon when all the Servants are on offer all at once or when a smaller number of them are? Does it hurt your chances of getting the specific Servant you want or is it all the same anyway and you just get a better chance of getting at least one of the banner's Servants when more are being offered up at the same time?
You always have only a 1% chance of getting a 5* servant per roll.
If there's a single servant on rate-up, you have a 0.7% chance of getting him (and a 0.3% chance of some random 5*).
If two 5* servants are on rate-up, each of them have a 0.4% chance (and a 0.2% chance of some random 5*).
If three 5* servants are on rate-up, each of them have--IIRC--a 0.3% chance (and a 0.1% chance of some random 5*).
It's never really a good idea to roll when multiples of a rarity are on at the same time unless you're okay with getting any of them. If you're after a specific servant, always go after them if they are the only servant of their rarity on rate-up at that time; ie if you want a 4* only pull for that 4* if there are no other 4*s on rate-up.