I think that there may not be any answer to this question - at least not a good one that includes objectively defining "regularity" without any context. Surely, some words including some combination(s) are especially common, looking at Japanese as a whole, and I'd hazard (from a subjective standpoint) that combinations with "ウ + small version vowel" as well as ティ are among those, but beyond that? Well, according to Wikipedia:
Small versions of the five vowel kana are sometimes used to represent
trailing off sounds (ハァ haa, ネェ nee), but in katakana they are more
often used in yōon-like extended digraphs designed to represent
phonemes not present in Japanese; examples include チェ (che) in チェンジ
chenji ("change"), and ウィ (wi) and ディ (di) in ウィキペディア Wikipedia.
I believe that this phenomenon could easily (but roughly) be categorized into two usage cases. The first would then be loanwords, where パーティー is a good example.
Slightly off topic, I think it's worth mentioning that while English phonemes not present in Japanese can be rather easily identified, the "rules" for spelling in katakana may be harder to discern. Looking at how "party" becomes パーティー with katakana, an unknowing student may be tempted to write "ticket" as ティケット, but alas, it's チケット. Certainly, there are tendencies (is it a leading, trailing, middle, surrounding phonemes etc.), but there is also some amount of discretion involved. (Why is a word beginning with [ˈka] or [ˈkæ] that could arguably be spelled カ as in カメラ, suddenly キャ, as in キャンパス?)
The second usage case I was thinking about is proper names. Naturally, there are many "fixed" spellings of given names for people, countries and such, for example ウィリアム, or スウェーデン to name but a few. The desired pronunciation, and hence katakana spelling, in the case of people may however be a matter of individual preference.
And by the way, not all loanwords are English in origin, of course. Even some that are easily mistaken for "English" are not, like タバコ (Portuguese), ビール (Dutch) and アレルギー (German). Sounding rules of the source language may also affect how a word is transcribed.
For example, my own middle name is Knut. In my native language the "k" is "hard", and the "n" is pronounced separately, giving roughly [kn'ʉːt] (in contrast to words like "knight" in English). When having this name transcribed for my Japanese student's ID (many years ago), they went with the English sounding rules, in this case giving ナット, which to me was... Not perfect.
Anyway, if you were to construct a list of foreign words in Japanese that included "extended digraphs", then took it upon you to study their frequency, then you'd probably find that some are rather more common then others, but as I've been saying, that's not all to consider. E.g. in the context of mahjong, where most loanwords are Chinese, you'd find both high frequencies and a practical use for one set of "extended digraphs" different from what is common in another.
Absent a context, "anything goes". If your question would have been "which extended katakana are there", many dictionaries and such include lists. Googling, I found this one on-line, that contains I think all combinations that I've ever seen, and some that I haven't seen before:
Katakana (including List of extended katakana)
I hope this helps.
Edit: Note that the chart in the list linked above, and others linked in other answers, may change from time to time.