I think it's fair to expect a representative amount of MPs to be trans, in the same way that it would be fair to expect around 50% of MPs to be female (in an ideal world). There are 650 MPs, and between 200,000 and 500,000 trans people in the UK according to this:
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/721642/GEO-LGBT-factsheet.pdf
A proportional number of trans MPs, if we take a conservative estimate of the number of trans people as 200,000, (seeing as we don't know how many included in that estimate are above the age of 18) would be: 200,000/68,000,000 * 650 = 1.9.
So 2 MPs for trans people would be the equivalent of hitting 50% representation for women. Any more than 2, and they would be over-represented.
The question is though, as trans can mean so many different things, what type of trans person would be elected into those positions. For example, can we really consider that a middle-aged AGP trans identifying male could represent well the interests of a young trans man? Trans is a meaningless term, because it includes such a wide variety of different profiles in terms of sex, age, and the type of 'identification'. Do we have quotas for religious groups, because it seems to make more sense to me to treat this as a religious belief rather than a specific identity as is race or sex.