Ah got it yeh. always find that a bit weird in reviews ''number of positions'' you're allowed to do but must be my ignorance. I see what you meant by standard service now 
I don't take a copy of the Kamasutra with me on bookings and say can we do pages 11, 105 and 58?
I generally just do missionary as it's usually a good starter and lets the SP's vagina open up and adjust to the size of your cock plus you can kiss while you're doing it. After that it's usually doggy as it's visually good and cowgirl or reverse cowgirl. Sometimes you can do variants like standing doggy or prone bone, but that's about it for the positions I do and I get the feeling it's pretty standard. I dont know the names of others and would struggle to physically align my bits in others like spooning, so I don't do them because we spend our time trying to get into some unusual position that doesn't work for me instead of shagging and I'm really there for the shagging not aerobics. I stick with what I know and what SPs are happy to do and is easy for everyone to get into the right position physically for.
You also have to remember that SPs are always mindful of their physical and sexual safety so they will be uncomfortable/wary of getting into positions with a new SS where they think they might be unsafe physically or can't control things or where they think the risk of sexual pain or damage or condoms breaking is high, so they may say no to a position.
Don't go believing everything you read in reviews and comparing yours to theirs. We all like different things. We allhave different levels of experience and knowledge of positions. Just do what works for you and what you enjoy.
Sometimes you'll see an SP who might suggest something new like rimming or CIM and say "do you want to try it?". It's up to you to say yes or no. However, I find most SPs stick with the tried and tested "standard" positions of missionary, doggy, cowgirl and a few variants. I've not had any suggest doing wheelbarrow and I don't take little wheels with me to bookings, so don't know how we'd do that without one. I don't really experiment with other positions and I think most SPs don't either.